creds-util: add infra for encrypting/decrypting credentials

1 files changed, 876 insertions, 0 deletions

diff --git a/src/shared/creds-util.c b/src/shared/creds-util.c
index 58076705e7..6c2d9dbc76 100644
--- a/src/shared/creds-util.c
+++ b/src/shared/creds-util.c
@@ -1,9 +1,30 @@
 /* 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
@@ -52,3 +73,858 @@ int read_credential(const char *name, void **ret, size_t *ret_size) {
                         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;
+
+                                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;
+        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]) {
+
+        SHA256_CTX sha256_context;
+
+        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. */
+
+        if (SHA256_Init(&sha256_context) != 1)
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Failed to initial SHA256 context.");
+
+        if (host_key && SHA256_Update(&sha256_context, host_key, host_key_size) != 1)
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Failed to hash host key.");
+
+        if (tpm2_key && SHA256_Update(&sha256_context, tpm2_key, tpm2_key_size) != 1)
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Failed to hash TPM2 key.");
+
+        if (SHA256_Final(ret, &sha256_context) != 1)
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Failed to finalize SHA256 hash.");
+
+        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;
+        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);
+                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->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) {
+                size_t base64_size;
+
+                base64_size = DIV_ROUND_UP(p * 4, 3); /* Include base64 size increase in debug output */
+
+                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 (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),
+                                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