creds-util: switch to OpenSSL 3.0 APIs

Let's switch from the low-level SHA256 APIs to EVP APIs. The former are
deprecated on OpenSSL 3.0, the latter are supported both by old
OpenSSL and by OpenSSL 3.0, hence are the better choice.

Fixes: #20775
(cherry picked from commit 18f568b8e64b48f6aee204cc6384b4796cd27eb0)

2 files changed, 955 insertions, 0 deletions

diff --git a/src/shared/creds-util.c b/src/shared/creds-util.c
new file mode 100644
index 0000000000..b764198b76
--- /dev/null
+++ b/src/shared/creds-util.c
@@ -0,0 +1,954 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include <sys/file.h>
+
+#if HAVE_OPENSSL
+#include <openssl/err.h>
+#endif
+
+#include "sd-id128.h"
+
+#include "blockdev-util.h"
+#include "chattr-util.h"
+#include "creds-util.h"
+#include "env-util.h"
+#include "fd-util.h"
+#include "fileio.h"
+#include "fs-util.h"
+#include "io-util.h"
+#include "memory-util.h"
+#include "mkdir.h"
+#include "openssl-util.h"
+#include "path-util.h"
+#include "random-util.h"
+#include "sparse-endian.h"
+#include "stat-util.h"
+#include "tpm2-util.h"
+#include "virt.h"
+
+bool credential_name_valid(const char *s) {
+        /* We want that credential names are both valid in filenames (since that's our primary way to pass
+         * them around) and as fdnames (which is how we might want to pass them around eventually) */
+        return filename_is_valid(s) && fdname_is_valid(s);
+}
+
+int get_credentials_dir(const char **ret) {
+        const char *e;
+
+        assert(ret);
+
+        e = secure_getenv("CREDENTIALS_DIRECTORY");
+        if (!e)
+                return -ENXIO;
+
+        if (!path_is_absolute(e) || !path_is_normalized(e))
+                return -EINVAL;
+
+        *ret = e;
+        return 0;
+}
+
+int read_credential(const char *name, void **ret, size_t *ret_size) {
+        _cleanup_free_ char *fn = NULL;
+        const char *d;
+        int r;
+
+        assert(ret);
+
+        if (!credential_name_valid(name))
+                return -EINVAL;
+
+        r = get_credentials_dir(&d);
+        if (r < 0)
+                return r;
+
+        fn = path_join(d, name);
+        if (!fn)
+                return -ENOMEM;
+
+        return read_full_file_full(
+                        AT_FDCWD, fn,
+                        UINT64_MAX, SIZE_MAX,
+                        READ_FULL_FILE_SECURE,
+                        NULL,
+                        (char**) ret, ret_size);
+}
+
+#if HAVE_OPENSSL
+
+#define CREDENTIAL_HOST_SECRET_SIZE 4096
+
+static const sd_id128_t credential_app_id =
+        SD_ID128_MAKE(d3,ac,ec,ba,0d,ad,4c,df,b8,c9,38,15,28,93,6c,58);
+
+struct credential_host_secret_format {
+        /* The hashed machine ID of the machine this belongs to. Why? We want to ensure that each machine
+         * gets its own secret, even if people forget to flush out this secret file. Hence we bind it to the
+         * machine ID, for which there's hopefully a better chance it will be flushed out. We use a hashed
+         * machine ID instead of the literal one, because it's trivial to, and it might be a good idea not
+         * being able to directly associate a secret key file with a host. */
+        sd_id128_t machine_id;
+
+        /* The actual secret key */
+        uint8_t data[CREDENTIAL_HOST_SECRET_SIZE];
+} _packed_;
+
+static int make_credential_host_secret(
+                int dfd,
+                const sd_id128_t machine_id,
+                const char *fn,
+                void **ret_data,
+                size_t *ret_size) {
+
+        struct credential_host_secret_format buf;
+        _cleanup_free_ char *t = NULL;
+        _cleanup_close_ int fd = -1;
+        int r;
+
+        assert(dfd >= 0);
+        assert(fn);
+
+        fd = openat(dfd, ".", O_CLOEXEC|O_WRONLY|O_TMPFILE, 0400);
+        if (fd < 0) {
+                log_debug_errno(errno, "Failed to create temporary credential file with O_TMPFILE, proceeding without: %m");
+
+                if (asprintf(&t, "credential.secret.%016" PRIx64, random_u64()) < 0)
+                        return -ENOMEM;
+
+                fd = openat(dfd, t, O_CLOEXEC|O_WRONLY|O_CREAT|O_EXCL|O_NOFOLLOW, 0400);
+                if (fd < 0)
+                        return -errno;
+        }
+
+        r = chattr_secret(fd, 0);
+        if (r < 0)
+                log_debug_errno(r, "Failed to set file attributes for secrets file, ignoring: %m");
+
+        buf = (struct credential_host_secret_format) {
+                .machine_id = machine_id,
+        };
+
+        r = genuine_random_bytes(buf.data, sizeof(buf.data), RANDOM_BLOCK);
+        if (r < 0)
+                goto finish;
+
+        r = loop_write(fd, &buf, sizeof(buf), false);
+        if (r < 0)
+                goto finish;
+
+        if (fsync(fd) < 0) {
+                r = -errno;
+                goto finish;
+        }
+
+        if (t) {
+                r = rename_noreplace(dfd, t, dfd, fn);
+                if (r < 0)
+                        goto finish;
+
+                t = mfree(t);
+        } else if (linkat(fd, "", dfd, fn, AT_EMPTY_PATH) < 0) {
+                r = -errno;
+                goto finish;
+        }
+
+        if (fsync(dfd) < 0) {
+                r = -errno;
+                goto finish;
+        }
+
+        if (ret_data) {
+                void *copy;
+
+                copy = memdup(buf.data, sizeof(buf.data));
+                if (!copy) {
+                        r = -ENOMEM;
+                        goto finish;
+                }
+
+                *ret_data = copy;
+        }
+
+        if (ret_size)
+                *ret_size = sizeof(buf.data);
+
+        r = 0;
+
+finish:
+        if (t && unlinkat(dfd, t, 0) < 0)
+                log_debug_errno(errno, "Failed to remove temporary credential key: %m");
+
+        explicit_bzero_safe(&buf, sizeof(buf));
+        return r;
+}
+
+int get_credential_host_secret(CredentialSecretFlags flags, void **ret, size_t *ret_size) {
+        _cleanup_free_ char *efn = NULL, *ep = NULL;
+        _cleanup_close_ int dfd = -1;
+        sd_id128_t machine_id;
+        const char *e, *fn, *p;
+        int r;
+
+        r = sd_id128_get_machine_app_specific(credential_app_id, &machine_id);
+        if (r < 0)
+                return r;
+
+        e = secure_getenv("SYSTEMD_CREDENTIAL_SECRET");
+        if (e) {
+                if (!path_is_normalized(e))
+                        return -EINVAL;
+                if (!path_is_absolute(e))
+                        return -EINVAL;
+
+                r = path_extract_directory(e, &ep);
+                if (r < 0)
+                        return r;
+
+                r = path_extract_filename(e, &efn);
+                if (r < 0)
+                        return r;
+
+                p = ep;
+                fn = efn;
+        } else {
+                p = "/var/lib/systemd";
+                fn = "credential.secret";
+        }
+
+        (void) mkdir_p(p, 0755);
+        dfd = open(p, O_CLOEXEC|O_DIRECTORY|O_RDONLY);
+        if (dfd < 0)
+                return -errno;
+
+        if (FLAGS_SET(flags, CREDENTIAL_SECRET_FAIL_ON_TEMPORARY_FS)) {
+                r = fd_is_temporary_fs(dfd);
+                if (r < 0)
+                        return r;
+                if (r > 0)
+                        return -ENOMEDIUM;
+        }
+
+        for (unsigned attempt = 0;; attempt++) {
+                _cleanup_(erase_and_freep) struct credential_host_secret_format *f = NULL;
+                _cleanup_close_ int fd = -1;
+                size_t l = 0;
+                ssize_t n = 0;
+                struct stat st;
+
+                if (attempt >= 3) /* Somebody is playing games with us */
+                        return -EIO;
+
+                fd = openat(dfd, fn, O_CLOEXEC|O_RDONLY|O_NOCTTY|O_NOFOLLOW);
+                if (fd < 0) {
+                        if (errno != ENOENT || !FLAGS_SET(flags, CREDENTIAL_SECRET_GENERATE))
+                                return -errno;
+
+                        r = make_credential_host_secret(dfd, machine_id, fn, ret, ret_size);
+                        if (r == -EEXIST) {
+                                log_debug_errno(r, "Credential secret was created while we were creating it. Trying to read new secret.");
+                                continue;
+                        }
+                        if (r < 0)
+                                return r;
+
+                        return 0;
+                }
+
+                if (fstat(fd, &st) < 0)
+                        return -errno;
+
+                r = stat_verify_regular(&st);
+                if (r < 0)
+                        return r;
+                if (st.st_nlink == 0) /* Deleted by now, try again */
+                        continue;
+                if (st.st_nlink > 1)
+                        return -EPERM; /* Our deletion check won't work if hardlinked somewhere else */
+                if ((st.st_mode & 07777) != 0400) /* Don't use file if not 0400 access mode */
+                        return -EPERM;
+                if (st.st_size > 16*1024*1024)
+                        return -E2BIG;
+                l = st.st_size;
+                if (l < offsetof(struct credential_host_secret_format, data) + 1)
+                        return -EINVAL;
+
+                f = malloc(l+1);
+                if (!f)
+                        return -ENOMEM;
+
+                n = read(fd, f, l+1);
+                if (n < 0)
+                        return -errno;
+                if ((size_t) n != l) /* What? The size changed? */
+                        return -EIO;
+
+                if (sd_id128_equal(machine_id, f->machine_id)) {
+                        size_t sz;
+
+                        if (FLAGS_SET(flags, CREDENTIAL_SECRET_WARN_NOT_ENCRYPTED)) {
+                                r = fd_is_encrypted(fd);
+                                if (r < 0)
+                                        log_debug_errno(r, "Failed to determine if credential secret file '%s/%s' is encrypted.", p, fn);
+                                else if (r == 0)
+                                        log_warning("Credential secret file '%s/%s' is not located on encrypted media, using anyway.", p, fn);
+                        }
+
+                        sz = l - offsetof(struct credential_host_secret_format, data);
+                        assert(sz > 0);
+
+                        if (ret) {
+                                void *copy;
+
+                                assert(sz <= sizeof(f->data)); /* Ensure we don't read past f->data bounds */
+
+                                copy = memdup(f->data, sz);
+                                if (!copy)
+                                        return -ENOMEM;
+
+                                *ret = copy;
+                        }
+
+                        if (ret_size)
+                                *ret_size = sz;
+
+                        return 0;
+                }
+
+                /* Hmm, this secret is from somewhere else. Let's delete the file. Let's first acquire a lock
+                 * to ensure we are the only ones accessing the file while we delete it. */
+
+                if (flock(fd, LOCK_EX) < 0)
+                        return -errno;
+
+                /* Before we delete it check that the file is still linked into the file system */
+                if (fstat(fd, &st) < 0)
+                        return -errno;
+                if (st.st_nlink == 0) /* Already deleted by now? */
+                        continue;
+                if (st.st_nlink != 1) /* Safety check, someone is playing games with us */
+                        return -EPERM;
+
+                if (unlinkat(dfd, fn, 0) < 0)
+                        return -errno;
+
+                /* And now try again */
+        }
+}
+
+/* Construction is like this:
+ *
+ * A symmetric encryption key is derived from:
+ *
+ *      1. Either the "host" key (a key stored in /var/lib/credential.secret)
+ *
+ *      2. A key generated by letting the TPM2 calculate an HMAC hash of some nonce we pass to it, keyed
+ *         by a key derived from its internal seed key.
+ *
+ *      3. The concatenation of the above.
+ *
+ * The above is hashed with SHA256 which is then used as encryption key for AES256-GCM. The encrypted
+ * credential is a short (unencrypted) header describing which of the three keys to use, the IV to use for
+ * AES256-GCM and some more meta information (sizes of certain objects) that is strictly speaking redundant,
+ * but kinda nice to have since we can have a more generic parser. If the TPM2 key is used this is followed
+ * by another (unencrypted) header, with information about the TPM2 policy used (specifically: the PCR mask
+ * to bind against, and a hash of the resulting policy — the latter being redundant, but speeding up things a
+ * bit, since we can more quickly refuse PCR state), followed by a sealed/exported TPM2 HMAC key. This is
+ * then followed by the encrypted data, which begins with a metadata header (which contains validity
+ * timestamps as well as the credential name), followed by the actual credential payload. The file ends in
+ * the AES256-GCM tag. To make things simple, the AES256-GCM AAD covers the main and the TPM2 header in
+ * full. This means the whole file is either protected by AAD, or is ciphertext, or is the tag. No
+ * unprotected data is included.
+ */
+
+struct _packed_ encrypted_credential_header {
+        sd_id128_t id;
+        le32_t key_size;
+        le32_t block_size;
+        le32_t iv_size;
+        le32_t tag_size;
+        uint8_t iv[];
+        /* Followed by NUL bytes until next 8 byte boundary */
+};
+
+struct _packed_ tpm2_credential_header {
+        le64_t pcr_mask;    /* Note that the spec for PC Clients only mandates 24 PCRs, and that's what systems
+                             * generally have. But keep the door open for more. */
+        le16_t pcr_bank;    /* For now, either TPM2_ALG_SHA256 or TPM2_ALG_SHA1 */
+        le16_t primary_alg; /* Primary key algorithm (either TPM2_ALG_RSA or TPM2_ALG_ECC for now) */
+        le32_t blob_size;
+        le32_t policy_hash_size;
+        uint8_t policy_hash_and_blob[];
+        /* Followed by NUL bytes until next 8 byte boundary */
+};
+
+struct _packed_ metadata_credential_header {
+        le64_t timestamp;
+        le64_t not_after;
+        le32_t name_size;
+        char name[];
+        /* Followed by NUL bytes until next 8 byte boundary */
+};
+
+/* Some generic limit for parts of the encrypted credential for which we don't know the right size ahead of
+ * time, but where we are really sure it won't be larger than this. Should be larger than any possible IV,
+ * padding, tag size and so on. This is purely used for early filtering out of invalid sizes. */
+#define CREDENTIAL_FIELD_SIZE_MAX (16U*1024U)
+
+static int sha256_hash_host_and_tpm2_key(
+                const void *host_key,
+                size_t host_key_size,
+                const void *tpm2_key,
+                size_t tpm2_key_size,
+                uint8_t ret[static SHA256_DIGEST_LENGTH]) {
+
+        _cleanup_(EVP_MD_CTX_freep) EVP_MD_CTX *md = NULL;
+        unsigned l;
+
+        assert(host_key_size == 0 || host_key);
+        assert(tpm2_key_size == 0 || tpm2_key);
+        assert(ret);
+
+        /* Combines the host key and the TPM2 HMAC hash into a SHA256 hash value we'll use as symmetric encryption key. */
+
+        md = EVP_MD_CTX_new();
+        if (!md)
+                return log_oom();
+
+        if (EVP_DigestInit_ex(md, EVP_sha256(), NULL) != 1)
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Failed to initial SHA256 context.");
+
+        if (host_key && EVP_DigestUpdate(md, host_key, host_key_size) != 1)
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Failed to hash host key.");
+
+        if (tpm2_key && EVP_DigestUpdate(md, tpm2_key, tpm2_key_size) != 1)
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Failed to hash TPM2 key.");
+
+        assert(EVP_MD_CTX_size(md) == SHA256_DIGEST_LENGTH);
+
+        if (EVP_DigestFinal_ex(md, ret, &l) != 1)
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Failed to finalize SHA256 hash.");
+
+        assert(l == SHA256_DIGEST_LENGTH);
+        return 0;
+}
+
+int encrypt_credential_and_warn(
+                sd_id128_t with_key,
+                const char *name,
+                usec_t timestamp,
+                usec_t not_after,
+                const char *tpm2_device,
+                uint32_t tpm2_pcr_mask,
+                const void *input,
+                size_t input_size,
+                void **ret,
+                size_t *ret_size) {
+
+        _cleanup_(EVP_CIPHER_CTX_freep) EVP_CIPHER_CTX *context = NULL;
+        _cleanup_(erase_and_freep) void *host_key = NULL, *tpm2_key = NULL;
+        size_t host_key_size = 0, tpm2_key_size = 0, tpm2_blob_size = 0, tpm2_policy_hash_size = 0, output_size, p, ml;
+        _cleanup_free_ void *tpm2_blob = NULL, *tpm2_policy_hash = NULL, *iv = NULL, *output = NULL;
+        _cleanup_free_ struct metadata_credential_header *m = NULL;
+        uint16_t tpm2_pcr_bank = 0, tpm2_primary_alg = 0;
+        struct encrypted_credential_header *h;
+        int ksz, bsz, ivsz, tsz, added, r;
+        uint8_t md[SHA256_DIGEST_LENGTH];
+        const EVP_CIPHER *cc;
+#if HAVE_TPM2
+        bool try_tpm2 = false;
+#endif
+        sd_id128_t id;
+
+        assert(input || input_size == 0);
+        assert(ret);
+        assert(ret_size);
+
+        if (name && !credential_name_valid(name))
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Invalid credential name: %s", name);
+
+        if (not_after != USEC_INFINITY && timestamp != USEC_INFINITY && not_after < timestamp)
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Credential is invalidated before it is valid (" USEC_FMT " < " USEC_FMT ").", not_after, timestamp);
+
+        if (DEBUG_LOGGING) {
+                char buf[FORMAT_TIMESTAMP_MAX];
+
+                if (name)
+                        log_debug("Including credential name '%s' in encrypted credential.", name);
+                if (timestamp != USEC_INFINITY)
+                        log_debug("Including timestamp '%s' in encrypted credential.", format_timestamp(buf, sizeof(buf), timestamp));
+                if (not_after != USEC_INFINITY)
+                        log_debug("Including not-after timestamp '%s' in encrypted credential.", format_timestamp(buf, sizeof(buf), not_after));
+        }
+
+        if (sd_id128_is_null(with_key) ||
+            sd_id128_in_set(with_key, CRED_AES256_GCM_BY_HOST, CRED_AES256_GCM_BY_HOST_AND_TPM2_HMAC)) {
+
+                r = get_credential_host_secret(
+                                CREDENTIAL_SECRET_GENERATE|
+                                CREDENTIAL_SECRET_WARN_NOT_ENCRYPTED|
+                                (sd_id128_is_null(with_key) ? CREDENTIAL_SECRET_FAIL_ON_TEMPORARY_FS : 0),
+                                &host_key,
+                                &host_key_size);
+                if (r == -ENOMEDIUM && sd_id128_is_null(with_key))
+                        log_debug_errno(r, "Credential host secret location on temporary file system, not using.");
+                else if (r < 0)
+                        return log_error_errno(r, "Failed to determine local credential host secret: %m");
+        }
+
+#if HAVE_TPM2
+        if (sd_id128_is_null(with_key)) {
+                /* If automatic mode is selected and we are running in a container, let's not try TPM2. OTOH
+                 * if user picks TPM2 explicitly, let's always honour the request and try. */
+
+                r = detect_container();
+                if (r < 0)
+                        log_debug_errno(r, "Failed to determine whether we are running in a container, ignoring: %m");
+                else if (r > 0)
+                        log_debug("Running in container, not attempting to use TPM2.");
+
+                try_tpm2 = r <= 0;
+        }
+
+        if (try_tpm2 ||
+            sd_id128_in_set(with_key, CRED_AES256_GCM_BY_TPM2_HMAC, CRED_AES256_GCM_BY_HOST_AND_TPM2_HMAC)) {
+
+                r = tpm2_seal(tpm2_device,
+                              tpm2_pcr_mask,
+                              &tpm2_key,
+                              &tpm2_key_size,
+                              &tpm2_blob,
+                              &tpm2_blob_size,
+                              &tpm2_policy_hash,
+                              &tpm2_policy_hash_size,
+                              &tpm2_pcr_bank,
+                              &tpm2_primary_alg);
+                if (r < 0) {
+                        if (!sd_id128_is_null(with_key))
+                                return r;
+
+                        log_debug_errno(r, "TPM2 sealing didn't work, not using: %m");
+                }
+
+                assert(tpm2_blob_size <= CREDENTIAL_FIELD_SIZE_MAX);
+                assert(tpm2_policy_hash_size <= CREDENTIAL_FIELD_SIZE_MAX);
+        }
+#endif
+
+        if (sd_id128_is_null(with_key)) {
+                /* Let's settle the key type in auto mode now. */
+
+                if (host_key && tpm2_key)
+                        id = CRED_AES256_GCM_BY_HOST_AND_TPM2_HMAC;
+                else if (tpm2_key)
+                        id = CRED_AES256_GCM_BY_TPM2_HMAC;
+                else if (host_key)
+                        id = CRED_AES256_GCM_BY_HOST;
+                else
+                        return log_error_errno(SYNTHETIC_ERRNO(ENOTRECOVERABLE),
+                                               "TPM2 not available and host key located on temporary file system, no encryption key available.");
+        } else
+                id = with_key;
+
+        /* Let's now take the host key and the TPM2 key and hash it together, to use as encryption key for the data */
+        r = sha256_hash_host_and_tpm2_key(host_key, host_key_size, tpm2_key, tpm2_key_size, md);
+        if (r < 0)
+                return r;
+
+        assert_se(cc = EVP_aes_256_gcm());
+
+        ksz = EVP_CIPHER_key_length(cc);
+        assert(ksz == sizeof(md));
+
+        bsz = EVP_CIPHER_block_size(cc);
+        assert(bsz > 0);
+        assert((size_t) bsz <= CREDENTIAL_FIELD_SIZE_MAX);
+
+        ivsz = EVP_CIPHER_iv_length(cc);
+        if (ivsz > 0) {
+                assert((size_t) ivsz <= CREDENTIAL_FIELD_SIZE_MAX);
+
+                iv = malloc(ivsz);
+                if (!iv)
+                        return log_oom();
+
+                r = genuine_random_bytes(iv, ivsz, RANDOM_BLOCK);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to acquired randomized IV: %m");
+        }
+
+        tsz = 16; /* FIXME: On OpenSSL 3 there is EVP_CIPHER_CTX_get_tag_length(), until then let's hardcode this */
+
+        context = EVP_CIPHER_CTX_new();
+        if (!context)
+                return log_error_errno(SYNTHETIC_ERRNO(ENOMEM), "Failed to allocate encryption object: %s",
+                                       ERR_error_string(ERR_get_error(), NULL));
+
+        if (EVP_EncryptInit_ex(context, cc, NULL, md, iv) != 1)
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Failed to initialize encryption context: %s",
+                                       ERR_error_string(ERR_get_error(), NULL));
+
+        /* Just an upper estimate */
+        output_size =
+                ALIGN8(offsetof(struct encrypted_credential_header, iv) + ivsz) +
+                ALIGN8(tpm2_key ? offsetof(struct tpm2_credential_header, policy_hash_and_blob) + tpm2_blob_size + tpm2_policy_hash_size : 0) +
+                ALIGN8(offsetof(struct metadata_credential_header, name) + strlen_ptr(name)) +
+                input_size + 2U * (size_t) bsz +
+                tsz;
+
+        output = malloc0(output_size);
+        if (!output)
+                return log_oom();
+
+        h = (struct encrypted_credential_header*) output;
+        h->id = id;
+        h->block_size = htole32(bsz);
+        h->key_size = htole32(ksz);
+        h->tag_size = htole32(tsz);
+        h->iv_size = htole32(ivsz);
+        memcpy(h->iv, iv, ivsz);
+
+        p = ALIGN8(offsetof(struct encrypted_credential_header, iv) + ivsz);
+
+        if (tpm2_key) {
+                struct tpm2_credential_header *t;
+
+                t = (struct tpm2_credential_header*) ((uint8_t*) output + p);
+                t->pcr_mask = htole64(tpm2_pcr_mask);
+                t->pcr_bank = htole16(tpm2_pcr_bank);
+                t->primary_alg = htole16(tpm2_primary_alg);
+                t->blob_size = htole32(tpm2_blob_size);
+                t->policy_hash_size = htole32(tpm2_policy_hash_size);
+                memcpy(t->policy_hash_and_blob, tpm2_blob, tpm2_blob_size);
+                memcpy(t->policy_hash_and_blob + tpm2_blob_size, tpm2_policy_hash, tpm2_policy_hash_size);
+
+                p += ALIGN8(offsetof(struct tpm2_credential_header, policy_hash_and_blob) + tpm2_blob_size + tpm2_policy_hash_size);
+        }
+
+        /* Pass the encrypted + TPM2 header as AAD */
+        if (EVP_EncryptUpdate(context, NULL, &added, output, p) != 1)
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Failed to write AAD data: %s",
+                                       ERR_error_string(ERR_get_error(), NULL));
+
+        /* Now construct the metadata header */
+        ml = strlen_ptr(name);
+        m = malloc0(ALIGN8(offsetof(struct metadata_credential_header, name) + ml));
+        if (!m)
+                return log_oom();
+
+        m->timestamp = htole64(timestamp);
+        m->not_after = htole64(not_after);
+        m->name_size = htole32(ml);
+        memcpy_safe(m->name, name, ml);
+
+        /* And encrypt the metadata header */
+        if (EVP_EncryptUpdate(context, (uint8_t*) output + p, &added, (const unsigned char*) m, ALIGN8(offsetof(struct metadata_credential_header, name) + ml)) != 1)
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Failed to encrypt metadata header: %s",
+                                       ERR_error_string(ERR_get_error(), NULL));
+
+        assert(added >= 0);
+        assert((size_t) added <= output_size - p);
+        p += added;
+
+        /* Then encrypt the plaintext */
+        if (EVP_EncryptUpdate(context, (uint8_t*) output + p, &added, input, input_size) != 1)
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Failed to encrypt data: %s",
+                                       ERR_error_string(ERR_get_error(), NULL));
+
+        assert(added >= 0);
+        assert((size_t) added <= output_size - p);
+        p += added;
+
+        /* Finalize */
+        if (EVP_EncryptFinal_ex(context, (uint8_t*) output + p, &added) != 1)
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Failed to finalize data encryption: %s",
+                                       ERR_error_string(ERR_get_error(), NULL));
+
+        assert(added >= 0);
+        assert((size_t) added <= output_size - p);
+        p += added;
+
+        assert(p <= output_size - tsz);
+
+        /* Append tag */
+        if (EVP_CIPHER_CTX_ctrl(context, EVP_CTRL_GCM_GET_TAG, tsz, (uint8_t*) output + p) != 1)
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Failed to get tag: %s",
+                                       ERR_error_string(ERR_get_error(), NULL));
+
+        p += tsz;
+        assert(p <= output_size);
+
+        if (DEBUG_LOGGING && input_size > 0) {
+                size_t base64_size;
+
+                base64_size = DIV_ROUND_UP(p * 4, 3); /* Include base64 size increase in debug output */
+                assert(base64_size >= input_size);
+                log_debug("Input of %zu bytes grew to output of %zu bytes (+%2zu%%).", input_size, base64_size, base64_size * 100 / input_size - 100);
+        }
+
+        *ret = TAKE_PTR(output);
+        *ret_size = p;
+
+        return 0;
+}
+
+int decrypt_credential_and_warn(
+                const char *validate_name,
+                usec_t validate_timestamp,
+                const char *tpm2_device,
+                const void *input,
+                size_t input_size,
+                void **ret,
+                size_t *ret_size) {
+
+        _cleanup_(erase_and_freep) void *host_key = NULL, *tpm2_key = NULL, *plaintext = NULL;
+        _cleanup_(EVP_CIPHER_CTX_freep) EVP_CIPHER_CTX *context = NULL;
+        size_t host_key_size = 0, tpm2_key_size = 0, plaintext_size, p, hs;
+        struct encrypted_credential_header *h;
+        struct metadata_credential_header *m;
+        uint8_t md[SHA256_DIGEST_LENGTH];
+        bool with_tpm2, with_host_key;
+        const EVP_CIPHER *cc;
+        int r, added;
+
+        assert(input || input_size == 0);
+        assert(ret);
+        assert(ret_size);
+
+        h = (struct encrypted_credential_header*) input;
+
+        /* The ID must fit in, for the current and all future formats */
+        if (input_size < sizeof(h->id))
+                return log_error_errno(SYNTHETIC_ERRNO(EBADMSG), "Encrypted file too short.");
+
+        with_host_key = sd_id128_in_set(h->id, CRED_AES256_GCM_BY_HOST, CRED_AES256_GCM_BY_HOST_AND_TPM2_HMAC);
+        with_tpm2 = sd_id128_in_set(h->id, CRED_AES256_GCM_BY_TPM2_HMAC, CRED_AES256_GCM_BY_HOST_AND_TPM2_HMAC);
+
+        if (!with_host_key && !with_tpm2)
+                return log_error_errno(SYNTHETIC_ERRNO(EOPNOTSUPP), "Unknown encryption format, or corrupted data: %m");
+
+        /* Now we know the minimum header size */
+        if (input_size < offsetof(struct encrypted_credential_header, iv))
+                return log_error_errno(SYNTHETIC_ERRNO(EBADMSG), "Encrypted file too short.");
+
+        /* Verify some basic header values */
+        if (le32toh(h->key_size) != sizeof(md))
+                return log_error_errno(SYNTHETIC_ERRNO(EBADMSG), "Unexpected key size in header.");
+        if (le32toh(h->block_size) <= 0 || le32toh(h->block_size) > CREDENTIAL_FIELD_SIZE_MAX)
+                return log_error_errno(SYNTHETIC_ERRNO(EBADMSG), "Unexpected block size in header.");
+        if (le32toh(h->iv_size) > CREDENTIAL_FIELD_SIZE_MAX)
+                return log_error_errno(SYNTHETIC_ERRNO(EBADMSG), "IV size too large.");
+        if (le32toh(h->tag_size) != 16) /* FIXME: On OpenSSL 3, let's verify via EVP_CIPHER_CTX_get_tag_length() */
+                return log_error_errno(SYNTHETIC_ERRNO(EBADMSG), "Unexpected tag size in header.");
+
+        /* Ensure we have space for the full header now (we don't know the size of the name hence this is a
+         * lower limit only) */
+        if (input_size <
+            ALIGN8(offsetof(struct encrypted_credential_header, iv) + le32toh(h->iv_size)) +
+            ALIGN8((with_tpm2 ? offsetof(struct tpm2_credential_header, policy_hash_and_blob) : 0)) +
+            ALIGN8(offsetof(struct metadata_credential_header, name)) +
+            le32toh(h->tag_size))
+                return log_error_errno(SYNTHETIC_ERRNO(EBADMSG), "Encrypted file too short.");
+
+        p = ALIGN8(offsetof(struct encrypted_credential_header, iv) + le32toh(h->iv_size));
+
+        if (with_tpm2) {
+#if HAVE_TPM2
+                struct tpm2_credential_header* t = (struct tpm2_credential_header*) ((uint8_t*) input + p);
+
+                if (le64toh(t->pcr_mask) >= (UINT64_C(1) << TPM2_PCRS_MAX))
+                        return log_error_errno(SYNTHETIC_ERRNO(EBADMSG), "TPM2 PCR mask out of range.");
+                if (!tpm2_pcr_bank_to_string(le16toh(t->pcr_bank)))
+                        return log_error_errno(SYNTHETIC_ERRNO(EBADMSG), "TPM2 PCR bank invalid or not supported");
+                if (!tpm2_primary_alg_to_string(le16toh(t->primary_alg)))
+                        return log_error_errno(SYNTHETIC_ERRNO(EBADMSG), "TPM2 primary key algorithm invalid or not supported.");
+                if (le32toh(t->blob_size) > CREDENTIAL_FIELD_SIZE_MAX)
+                        return log_error_errno(SYNTHETIC_ERRNO(EBADMSG), "Unexpected TPM2 blob size.");
+                if (le32toh(t->policy_hash_size) > CREDENTIAL_FIELD_SIZE_MAX)
+                        return log_error_errno(SYNTHETIC_ERRNO(EBADMSG), "Unexpected TPM2 policy hash size.");
+
+                /* Ensure we have space for the full TPM2 header now (still don't know the name, and its size
+                 * though, hence still just a lower limit test only) */
+                if (input_size <
+                    ALIGN8(offsetof(struct encrypted_credential_header, iv) + le32toh(h->iv_size)) +
+                    ALIGN8(offsetof(struct tpm2_credential_header, policy_hash_and_blob) + le32toh(t->blob_size) + le32toh(t->policy_hash_size)) +
+                    ALIGN8(offsetof(struct metadata_credential_header, name)) +
+                    le32toh(h->tag_size))
+                        return log_error_errno(SYNTHETIC_ERRNO(EBADMSG), "Encrypted file too short.");
+
+                r = tpm2_unseal(tpm2_device,
+                                le64toh(t->pcr_mask),
+                                le16toh(t->pcr_bank),
+                                le16toh(t->primary_alg),
+                                t->policy_hash_and_blob,
+                                le32toh(t->blob_size),
+                                t->policy_hash_and_blob + le32toh(t->blob_size),
+                                le32toh(t->policy_hash_size),
+                                &tpm2_key,
+                                &tpm2_key_size);
+                if (r < 0)
+                        return r;
+
+                p += ALIGN8(offsetof(struct tpm2_credential_header, policy_hash_and_blob) +
+                            le32toh(t->blob_size) +
+                            le32toh(t->policy_hash_size));
+#else
+                return log_error_errno(SYNTHETIC_ERRNO(EOPNOTSUPP), "Credential requires TPM2 support, but TPM2 support not available.");
+#endif
+        }
+
+        if (with_host_key) {
+                r = get_credential_host_secret(
+                                0,
+                                &host_key,
+                                &host_key_size);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to determine local credential key: %m");
+        }
+
+        sha256_hash_host_and_tpm2_key(host_key, host_key_size, tpm2_key, tpm2_key_size, md);
+
+        assert_se(cc = EVP_aes_256_gcm());
+
+        /* Make sure cipher expectations match the header */
+        if (EVP_CIPHER_key_length(cc) != (int) le32toh(h->key_size))
+                return log_error_errno(SYNTHETIC_ERRNO(EBADMSG), "Unexpected key size in header.");
+        if (EVP_CIPHER_block_size(cc) != (int) le32toh(h->block_size))
+                return log_error_errno(SYNTHETIC_ERRNO(EBADMSG), "Unexpected block size in header.");
+
+        context = EVP_CIPHER_CTX_new();
+        if (!context)
+                return log_error_errno(SYNTHETIC_ERRNO(ENOMEM), "Failed to allocate decryption object: %s",
+                                       ERR_error_string(ERR_get_error(), NULL));
+
+        if (EVP_DecryptInit_ex(context, cc, NULL, NULL, NULL) != 1)
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Failed to initialize decryption context: %s",
+                                       ERR_error_string(ERR_get_error(), NULL));
+
+        if (EVP_CIPHER_CTX_ctrl(context, EVP_CTRL_GCM_SET_IVLEN, le32toh(h->iv_size), NULL) != 1)
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Failed to set IV size on decryption context: %s",
+                                       ERR_error_string(ERR_get_error(), NULL));
+
+        if (EVP_DecryptInit_ex(context, NULL, NULL, md, h->iv) != 1)
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Failed to set IV and key: %s",
+                                       ERR_error_string(ERR_get_error(), NULL));
+
+        if (EVP_DecryptUpdate(context, NULL, &added, input, p) != 1)
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Failed to write AAD data: %s",
+                                       ERR_error_string(ERR_get_error(), NULL));
+
+        plaintext = malloc(input_size - p - le32toh(h->tag_size));
+        if (!plaintext)
+                return -ENOMEM;
+
+        if (EVP_DecryptUpdate(
+                            context,
+                            plaintext,
+                            &added,
+                            (uint8_t*) input + p,
+                            input_size - p - le32toh(h->tag_size)) != 1)
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Failed to decrypt data: %s",
+                                       ERR_error_string(ERR_get_error(), NULL));
+
+        assert(added >= 0);
+        assert((size_t) added <= input_size - p - le32toh(h->tag_size));
+        plaintext_size = added;
+
+        if (EVP_CIPHER_CTX_ctrl(context, EVP_CTRL_GCM_SET_TAG, le32toh(h->tag_size), (uint8_t*) input + input_size - le32toh(h->tag_size)) != 1)
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Failed to set tag: %s",
+                                       ERR_error_string(ERR_get_error(), NULL));
+
+        if (EVP_DecryptFinal_ex(context, (uint8_t*) plaintext + plaintext_size, &added) != 1)
+                return log_error_errno(SYNTHETIC_ERRNO(EBADMSG), "Decryption failed (incorrect key?): %s",
+                                       ERR_error_string(ERR_get_error(), NULL));
+
+        plaintext_size += added;
+
+        if (plaintext_size < ALIGN8(offsetof(struct metadata_credential_header, name)))
+                return log_error_errno(SYNTHETIC_ERRNO(EBADMSG), "Metadata header incomplete.");
+
+        m = plaintext;
+
+        if (le64toh(m->timestamp) != USEC_INFINITY &&
+            le64toh(m->not_after) != USEC_INFINITY &&
+            le64toh(m->timestamp) >= le64toh(m->not_after))
+                return log_error_errno(SYNTHETIC_ERRNO(EBADMSG), "Timestamps of credential are not in order, refusing.");
+
+        if (le32toh(m->name_size) > CREDENTIAL_NAME_MAX)
+                return log_error_errno(SYNTHETIC_ERRNO(EBADMSG), "Embedded credential name too long, refusing.");
+
+        hs = ALIGN8(offsetof(struct metadata_credential_header, name) + le32toh(m->name_size));
+        if (plaintext_size < hs)
+                return log_error_errno(SYNTHETIC_ERRNO(EBADMSG), "Metadata header incomplete.");
+
+        if (le32toh(m->name_size) > 0) {
+                _cleanup_free_ char *embedded_name = NULL;
+
+                if (memchr(m->name, 0, le32toh(m->name_size)))
+                        return log_error_errno(SYNTHETIC_ERRNO(EBADMSG), "Embedded credential name contains NUL byte, refusing.");
+
+                embedded_name = memdup_suffix0(m->name, le32toh(m->name_size));
+                if (!embedded_name)
+                        return log_oom();
+
+                if (!credential_name_valid(embedded_name))
+                        return log_error_errno(SYNTHETIC_ERRNO(EBADMSG), "Embedded credential name is not valid, refusing.");
+
+                if (validate_name && !streq(embedded_name, validate_name)) {
+
+                        r = getenv_bool_secure("SYSTEMD_CREDENTIAL_VALIDATE_NAME");
+                        if (r < 0 && r != -ENXIO)
+                                log_debug_errno(r, "Failed to parse $SYSTEMD_CREDENTIAL_VALIDATE_NAME: %m");
+                        if (r != 0)
+                                return log_error_errno(SYNTHETIC_ERRNO(EREMOTE), "Embedded credential name '%s' does not match filename '%s', refusing.", embedded_name, validate_name);
+
+                        log_debug("Embedded credential name '%s' does not match expected name '%s', but configured to use credential anyway.", embedded_name, validate_name);
+                }
+        }
+
+        if (validate_timestamp != USEC_INFINITY) {
+                if (le64toh(m->timestamp) != USEC_INFINITY && le64toh(m->timestamp) > validate_timestamp)
+                        log_debug("Credential timestamp is from the future, assuming clock skew.");
+
+                if (le64toh(m->not_after) != USEC_INFINITY && le64toh(m->not_after) < validate_timestamp) {
+
+                        r = getenv_bool_secure("SYSTEMD_CREDENTIAL_VALIDATE_NOT_AFTER");
+                        if (r < 0 && r != -ENXIO)
+                                log_debug_errno(r, "Failed to parse $SYSTEMD_CREDENTIAL_VALIDATE_NOT_AFTER: %m");
+                        if (r != 0)
+                                return log_error_errno(SYNTHETIC_ERRNO(ESTALE), "Credential's time passed, refusing to use.");
+
+                        log_debug("Credential not-after timestamp has passed, but configured to use credential anyway.");
+                }
+        }
+
+        if (ret) {
+                char *without_metadata;
+
+                without_metadata = memdup((uint8_t*) plaintext + hs, plaintext_size - hs);
+                if (!without_metadata)
+                        return log_oom();
+
+                *ret = without_metadata;
+        }
+
+        if (ret_size)
+                *ret_size = plaintext_size - hs;
+
+        return 0;
+}
+
+#else
+
+int get_credential_host_secret(CredentialSecretFlags flags, void **ret, size_t *ret_size) {
+        return log_error_errno(SYNTHETIC_ERRNO(EOPNOTSUPP), "Support for encrypted credentials not available.");
+}
+
+int encrypt_credential_and_warn(sd_id128_t with_key, const char *name, usec_t timestamp, usec_t not_after, const char *tpm2_device, uint32_t tpm2_pcr_mask, const void *input, size_t input_size, void **ret, size_t *ret_size) {
+        return log_error_errno(SYNTHETIC_ERRNO(EOPNOTSUPP), "Support for encrypted credentials not available.");
+}
+
+int decrypt_credential_and_warn(const char *validate_name, usec_t validate_timestamp, const char *tpm2_device, const void *input, size_t input_size, void **ret, size_t *ret_size) {
+        return log_error_errno(SYNTHETIC_ERRNO(EOPNOTSUPP), "Support for encrypted credentials not available.");
+}
+
+#endif
diff --git a/src/shared/openssl-util.h b/src/shared/openssl-util.h
index e6c2bd9310..ce8207414f 100644
--- a/src/shared/openssl-util.h
+++ b/src/shared/openssl-util.h
@@ -11,6 +11,7 @@ DEFINE_TRIVIAL_CLEANUP_FUNC_FULL(X509*, X509_free, NULL);
 DEFINE_TRIVIAL_CLEANUP_FUNC_FULL(X509_NAME*, X509_NAME_free, NULL);
 DEFINE_TRIVIAL_CLEANUP_FUNC_FULL(EVP_PKEY_CTX*, EVP_PKEY_CTX_free, NULL);
 DEFINE_TRIVIAL_CLEANUP_FUNC_FULL(EVP_CIPHER_CTX*, EVP_CIPHER_CTX_free, NULL);
+DEFINE_TRIVIAL_CLEANUP_FUNC_FULL(EVP_MD_CTX*, EVP_MD_CTX_free, NULL);
 
 int rsa_encrypt_bytes(EVP_PKEY *pkey, const void *decrypted_key, size_t decrypted_key_size, void **ret_encrypt_key, size_t *ret_encrypt_key_size);