CR50: add support for RSA key "testing"

Implement _cpri__TestKeyRSA, which computes
the modulus and private exponent given a
pair of primes, or computes the second prime
and private exponent given the modulus and
one prime.

The _cpri__TestKeyRSA call is used to determine
whether the components of an RSA key match each other.

BRANCH=none
BUG=chrome-os-partner:43025,chrome-os-partner:47524
TEST=tests in test/tpm/tpmtest.py pass

Change-Id: I2c68d844f4bab207588cbda5c962b09078519a1a
Signed-off-by: nagendra modadugu <ngm@google.com>
Reviewed-on: https://chromium-review.googlesource.com/330466
Commit-Ready: Nagendra Modadugu <ngm@google.com>
Tested-by: Nagendra Modadugu <ngm@google.com>
Reviewed-by: Marius Schilder <mschilder@chromium.org>

7 files changed, 484 insertions, 20 deletions

diff --git a/board/cr50/tpm2/rsa.c b/board/cr50/tpm2/rsa.c
index 2e7ccd8208..e48e4fb352 100644
--- a/board/cr50/tpm2/rsa.c
+++ b/board/cr50/tpm2/rsa.c
@@ -206,6 +206,51 @@ CRYPT_RESULT _cpri__ValidateSignatureRSA(
 		return CRYPT_FAIL;
 }
 
+CRYPT_RESULT _cpri__TestKeyRSA(TPM2B *d_buf, uint32_t e,
+			TPM2B *N_buf, TPM2B *p_buf, TPM2B *q_buf)
+{
+	struct BIGNUM N;
+	struct BIGNUM p;
+	struct BIGNUM q;
+	struct BIGNUM d;
+	int result;
+
+	if (!p_buf)
+		return CRYPT_PARAMETER;
+	if (q_buf && p_buf->size != q_buf->size)
+		return CRYPT_PARAMETER;
+	if (N_buf->size != p_buf->size * 2)
+		return CRYPT_PARAMETER;  /* Insufficient output buffer space. */
+	if (N_buf->size > RSA_MAX_BYTES)
+		return CRYPT_PARAMETER;  /* Unsupported key size. */
+
+	DCRYPTO_bn_wrap(&N, N_buf->buffer, N_buf->size);
+	DCRYPTO_bn_wrap(&p, p_buf->buffer, p_buf->size);
+	reverse_tpm2b(N_buf);
+	reverse_tpm2b(p_buf);
+	if (q_buf) {
+		DCRYPTO_bn_wrap(&q, q_buf->buffer, q_buf->size);
+		reverse_tpm2b(q_buf);
+	}
+	/* d_buf->size may be uninitialized. */
+	DCRYPTO_bn_wrap(&d, d_buf->buffer, N_buf->size);
+
+	result = DCRYPTO_rsa_key_compute(&N, &d, &p, q_buf ? &q : NULL, e);
+
+	reverse_tpm2b(N_buf);
+	reverse_tpm2b(p_buf);
+	if (q_buf)
+		reverse_tpm2b(q_buf);
+
+	if (result) {
+		d_buf->size = N_buf->size;
+		reverse_tpm2b(d_buf);
+		return CRYPT_SUCCESS;
+	} else {
+		return CRYPT_FAIL;
+	}
+}
+
 #ifdef CRYPTO_TEST_SETUP
 
 #include "extension.h"
@@ -215,7 +260,8 @@ enum {
 	TEST_RSA_DECRYPT = 1,
 	TEST_RSA_SIGN = 2,
 	TEST_RSA_VERIFY = 3,
-	TEST_RSA_KEYGEN = 4
+	TEST_RSA_KEYGEN = 4,
+	TEST_RSA_KEYTEST = 5
 };
 
 static const TPM2B_PUBLIC_KEY_RSA RSA_768_N = {
@@ -254,6 +300,30 @@ static const TPM2B_PUBLIC_KEY_RSA RSA_768_D = {
 	}
 };
 
+static const TPM2B_PUBLIC_KEY_RSA RSA_768_P = {
+	.t = {48, {
+			0xd6, 0x09, 0x64, 0xc8, 0xf3, 0x5c, 0x02, 0xc7,
+			0xc6, 0x47, 0x4e, 0x7f, 0x43, 0x9d, 0x31, 0x46,
+			0x7a, 0x33, 0x85, 0xa0, 0xa4, 0x16, 0xea, 0x22,
+			0x7b, 0xcd, 0x64, 0x9b, 0x50, 0xec, 0xa7, 0x2f,
+			0x7e, 0xcf, 0xeb, 0x69, 0x29, 0x34, 0x8e, 0xb7,
+			0xb5, 0xb3, 0xba, 0x7f, 0x9b, 0x01, 0x7d, 0x69
+		}
+	}
+};
+
+static const TPM2B_PUBLIC_KEY_RSA RSA_768_Q = {
+	.t = {48, {
+			0xd3, 0x88, 0x92, 0x2d, 0xd5, 0xc6, 0x29, 0xf4,
+			0xf0, 0x2e, 0x61, 0xf0, 0x60, 0xad, 0xa9, 0x46,
+			0x11, 0xa9, 0x0c, 0x69, 0x14, 0x31, 0x09, 0x36,
+			0x8b, 0x70, 0x1b, 0x11, 0x9b, 0x26, 0x39, 0x34,
+			0x34, 0xfd, 0xf1, 0x9a, 0x89, 0x51, 0x63, 0x0a,
+			0xc6, 0x60, 0x0b, 0xba, 0x18, 0x8e, 0xc8, 0x01
+		}
+	}
+};
+
 static const TPM2B_PUBLIC_KEY_RSA RSA_2048_N = {
 	.t = {256, {
 			0x9c, 0xd7, 0x61, 0x2e, 0x43, 0x8e, 0x15, 0xbe,
@@ -330,6 +400,58 @@ static const TPM2B_PUBLIC_KEY_RSA RSA_2048_D = {
 	}
 };
 
+static const TPM2B_PUBLIC_KEY_RSA RSA_2048_P = {
+	.t  = {128, {
+			0xc8, 0x80, 0x6f, 0xf6, 0x2f, 0xfb, 0x49, 0x8b,
+			0x77, 0x39, 0xe2, 0x3d, 0x3d, 0x1f, 0x4d, 0xf9,
+			0xbb, 0x54, 0x06, 0x0d, 0x71, 0xbf, 0x54, 0xb1,
+			0x1e, 0xa2, 0x20, 0x7e, 0xdd, 0xcf, 0x21, 0x16,
+			0xe9, 0xc0, 0xba, 0x94, 0x02, 0xd2, 0xa4, 0x2e,
+			0x78, 0x3c, 0xfb, 0x64, 0xa0, 0xe7, 0xe9, 0x27,
+			0x64, 0x29, 0x19, 0x74, 0xc5, 0x77, 0xbb, 0xe1,
+			0x6d, 0xb4, 0x83, 0x1d, 0x43, 0x5a, 0x80, 0x72,
+			0xec, 0x3c, 0x32, 0xc3, 0x20, 0x2c, 0xce, 0xf7,
+			0xba, 0xf6, 0xc6, 0x0c, 0xf4, 0x56, 0xfd, 0xdf,
+			0x21, 0x55, 0xf3, 0xe2, 0x56, 0x25, 0xa6, 0xb3,
+			0x96, 0xa4, 0x9c, 0xb8, 0xfd, 0x9c, 0xec, 0x87,
+			0xfa, 0xda, 0x2e, 0xa4, 0xf6, 0x0f, 0x14, 0xe6,
+			0x81, 0x22, 0x84, 0xe7, 0xc0, 0x1d, 0xd1, 0x3f,
+			0xed, 0xb0, 0xba, 0xd8, 0xe4, 0xe9, 0xd4, 0x18,
+			0x33, 0xae, 0x29, 0x51, 0x79, 0x79, 0xd1, 0x0f
+		}
+	}
+};
+
+static const TPM2B_PUBLIC_KEY_RSA RSA_2048_Q = {
+	.t  = {128, {
+			0xc8, 0x41, 0x2a, 0x42, 0xf1, 0x6a, 0x81, 0xac,
+			0x06, 0xab, 0xd0, 0xb7, 0xc0, 0xbb, 0xc6, 0x13,
+			0xdd, 0xfd, 0x5e, 0x3c, 0x77, 0xfe, 0xc1, 0x2e,
+			0x76, 0xf0, 0x94, 0xc0, 0x5d, 0x24, 0x8b, 0x30,
+			0x0d, 0xf8, 0x2a, 0xc7, 0x26, 0x78, 0x1b, 0x81,
+			0x5a, 0x42, 0x96, 0xad, 0xf7, 0x0e, 0xa4, 0x1b,
+			0x2c, 0x8f, 0x38, 0x06, 0x05, 0x8d, 0x98, 0x6e,
+			0x37, 0x65, 0xb4, 0x2c, 0x80, 0xe2, 0x38, 0xd5,
+			0x79, 0xd2, 0xea, 0x62, 0xf2, 0x32, 0xac, 0x7b,
+			0x88, 0x90, 0xc3, 0x4e, 0x9e, 0x53, 0xe5, 0x7e,
+			0xef, 0x13, 0xb1, 0xe3, 0xd5, 0x41, 0xd1, 0xa9,
+			0x15, 0x04, 0x3c, 0x61, 0x74, 0x5e, 0x1a, 0x00,
+			0x5c, 0x8a, 0x8b, 0x17, 0xd5, 0x78, 0xad, 0x5e,
+			0xe0, 0xcf, 0x35, 0x63, 0x0a, 0x95, 0x1e, 0x70,
+			0xbe, 0x97, 0xf2, 0xd3, 0x78, 0x06, 0x8a, 0x88,
+			0x9b, 0x27, 0xc8, 0xb2, 0xb1, 0x3d, 0x8a, 0xd7
+		}
+	}
+};
+
+
+static const RSA_KEY RSA_768 = {
+	65537, (TPM2B *) &RSA_768_N.b, (TPM2B *) &RSA_768_D.b
+};
+static const RSA_KEY RSA_2048 = {
+	65537, (TPM2B *) &RSA_2048_N.b, (TPM2B *) &RSA_2048_D.b
+};
+
 #define MAX_MSG_BYTES RSA_MAX_BYTES
 
 static void rsa_command_handler(void *cmd_body,
@@ -348,8 +470,12 @@ static void rsa_command_handler(void *cmd_body,
 	uint8_t *out = (uint8_t *) cmd_body;
 	TPM2B_PUBLIC_KEY_RSA N;
 	TPM2B_PUBLIC_KEY_RSA d;
+	TPM2B_PUBLIC_KEY_RSA p;
+	TPM2B_PUBLIC_KEY_RSA q;
 	RSA_KEY key;
 	uint32_t *response_size = (uint32_t *) response_size_out;
+	TPM2B_PUBLIC_KEY_RSA rsa_d;
+	TPM2B_PUBLIC_KEY_RSA rsa_n;
 
 	assert(sizeof(size_t) == sizeof(uint32_t));
 
@@ -397,10 +523,18 @@ static void rsa_command_handler(void *cmd_body,
 	case 768:
 		N = RSA_768_N;
 		d = RSA_768_D;
+		p = RSA_768_P;
+		q = RSA_768_Q;
+		rsa_n.b.size = RSA_768_N.b.size;
+		rsa_d.b.size = RSA_768_D.b.size;
 		break;
 	case 2048:
 		N = RSA_2048_N;
 		d = RSA_2048_D;
+		p = RSA_2048_P;
+		q = RSA_2048_Q;
+		rsa_n.b.size = RSA_2048_N.b.size;
+		rsa_d.b.size = RSA_2048_D.b.size;
 		break;
 	default:
 		*response_size = 0;
@@ -446,6 +580,33 @@ static void rsa_command_handler(void *cmd_body,
 	case TEST_RSA_KEYGEN:
 		*response_size = 0;
 		break;
+	case TEST_RSA_KEYTEST:
+		if (_cpri__TestKeyRSA(&rsa_d.b, 65537, &rsa_n.b, &p.b, &q.b)
+			!= CRYPT_SUCCESS) {
+			*response_size = 0;
+			return;
+		}
+		if (memcmp(rsa_n.b.buffer, key.publicKey->buffer,
+				rsa_n.b.size) != 0 ||
+			memcmp(rsa_d.b.buffer, key.privateKey->buffer,
+					rsa_d.b.size) != 0) {
+				*response_size = 0;
+				return;
+		}
+
+		if (_cpri__TestKeyRSA(&rsa_d.b, 65537, key.publicKey,
+					&p.b, NULL) != CRYPT_SUCCESS) {
+			*response_size = 0;
+			return;
+		}
+		if (memcmp(rsa_d.b.buffer, key.privateKey->buffer,
+				rsa_d.b.size) != 0) {
+			*response_size = 0;
+			return;
+		}
+		*out = 1;
+		*response_size = 1;
+		return;
 	}
 }
 
diff --git a/board/cr50/tpm2/stubs.c b/board/cr50/tpm2/stubs.c
index 56465f7eae..257889a652 100644
--- a/board/cr50/tpm2/stubs.c
+++ b/board/cr50/tpm2/stubs.c
@@ -81,20 +81,6 @@ BOOL _cpri__Startup(
   return 1;
 }
 
-CRYPT_RESULT _cpri__TestKeyRSA(
-  TPM2B * d,                    //   OUT: the address to receive the
-  // private exponent
-  UINT32 exponent,              //   IN: the public modulu
-  TPM2B * publicKey,            //   IN/OUT: an input if only one prime is
-  // provided. an output if both primes are provided
-  TPM2B * prime1,               //   IN: a first prime
-  TPM2B * prime2                //   IN: an optional second prime
-  )
-{
-  ecprintf("%s called\n", __func__);
-  return CRYPT_FAIL;
-}
-
 CRYPT_RESULT _math__Div(
   const TPM2B * n,              //   IN: numerator
   const TPM2B * d,              //   IN: denominator
diff --git a/chip/g/dcrypto/bn.c b/chip/g/dcrypto/bn.c
index f4aaf11912..52a1abfc1a 100644
--- a/chip/g/dcrypto/bn.c
+++ b/chip/g/dcrypto/bn.c
@@ -16,10 +16,15 @@ static inline void watchdog_reload(void) { }
 
 void bn_init(struct BIGNUM *b, void *buf, size_t len)
 {
+	DCRYPTO_bn_wrap(b, buf, len);
+	dcrypto_memset(buf, 0x00, len);
+}
+
+void DCRYPTO_bn_wrap(struct BIGNUM *b, void *buf, size_t len)
+{
 	/* Only word-multiple sized buffers accepted. */
 	assert((len & 0x3) == 0);
 	b->dmax = len / BN_BYTES;
-	dcrypto_memset(buf, 0x00, len);
 	b->d = (struct access_helper *) buf;
 }
 
@@ -49,14 +54,26 @@ static int bn_is_bit_set(const struct BIGNUM *a, int n)
 static int bn_gte(const struct BIGNUM *a, const struct BIGNUM *b)
 {
 	int i;
+	uint32_t top = 0;
 
-	for (i = a->dmax - 1; BN_DIGIT(a, i) == BN_DIGIT(b, i) && i > 0; --i)
+	for (i = a->dmax - 1; i > b->dmax - 1; --i)
+		top |= BN_DIGIT(a, i);
+	if (top)
+		return 1;
+
+	for (i = b->dmax - 1; i > a->dmax - 1; --i)
+		top |= BN_DIGIT(b, i);
+	if (top)
+		return 0;
+
+	for (i = MIN(a->dmax, b->dmax) - 1;
+	     BN_DIGIT(a, i) == BN_DIGIT(b, i) && i > 0; --i)
 		;
 	return BN_DIGIT(a, i) >= BN_DIGIT(b, i);
 }
 
 /* c[] = c[] - a[], assumes c > a. */
-static uint32_t bn_sub(struct BIGNUM *c, const struct BIGNUM *a)
+uint32_t bn_sub(struct BIGNUM *c, const struct BIGNUM *a)
 {
 	int64_t A = 0;
 	int i;
@@ -66,11 +83,45 @@ static uint32_t bn_sub(struct BIGNUM *c, const struct BIGNUM *a)
 		BN_DIGIT(c, i) = (uint32_t) A;
 		A >>= 32;
 	}
+
+	for (; A && i < c->dmax; i++) {
+		A += (uint64_t) BN_DIGIT(c, i);
+		BN_DIGIT(c, i) = (uint32_t) A;
+		A >>= 32;
+	}
+
 	return (uint32_t) A;  /* 0 or -1. */
 }
 
+/* c[] = c[] - a[], negative numbers in 2's complement representation. */
+/* Returns borrow bit. */
+static uint32_t bn_signed_sub(struct BIGNUM *c, int *c_neg,
+		const struct BIGNUM *a, int a_neg)
+{
+	uint32_t carry = 0;
+	uint64_t A = 1;
+	int i;
+
+	for (i = 0; i < a->dmax; ++i) {
+		A += (uint64_t) BN_DIGIT(c, i) + ~BN_DIGIT(a, i);
+		BN_DIGIT(c, i) = (uint32_t) A;
+		A >>= 32;
+	}
+
+	for (; i < c->dmax; ++i) {
+		A += (uint64_t) BN_DIGIT(c, i) + 0xFFFFFFFF;
+		BN_DIGIT(c, i) = (uint32_t) A;
+		A >>= 32;
+	}
+
+	A &= 0x01;
+	carry = (!*c_neg && a_neg && A) || (*c_neg && !a_neg && !A);
+	*c_neg = carry ? *c_neg : (*c_neg + !a_neg + A) & 0x01;
+	return carry;
+}
+
 /* c[] = c[] + a[]. */
-static uint32_t bn_add(struct BIGNUM *c, const struct BIGNUM *a)
+uint32_t bn_add(struct BIGNUM *c, const struct BIGNUM *a)
 {
 	uint64_t A = 0;
 	int i;
@@ -81,9 +132,28 @@ static uint32_t bn_add(struct BIGNUM *c, const struct BIGNUM *a)
 		A >>= 32;
 	}
 
+	for (; A && i < c->dmax; ++i) {
+		A += (uint64_t) BN_DIGIT(c, i);
+		BN_DIGIT(c, i) = (uint32_t) A;
+		A >>= 32;
+	}
+
 	return (uint32_t) A;  /* 0 or 1. */
 }
 
+/* c[] = c[] + a[], negative numbers in 2's complement representation. */
+/* Returns carry bit. */
+static uint32_t bn_signed_add(struct BIGNUM *c, int *c_neg,
+			const struct BIGNUM *a, int a_neg)
+{
+	uint32_t A = bn_add(c, a);
+	uint32_t carry;
+
+	carry = (!*c_neg && !a_neg && A) || (*c_neg && a_neg && !A);
+	*c_neg = carry ? *c_neg : (*c_neg + a_neg + A) & 0x01;
+	return carry;
+}
+
 /* r[] <<= 1. */
 static uint32_t bn_lshift(struct BIGNUM *r)
 {
@@ -99,6 +169,29 @@ static uint32_t bn_lshift(struct BIGNUM *r)
 	return carry;
 }
 
+/* r[] >>= 1.  Handles 2's complement negative numbers. */
+static void bn_rshift(struct BIGNUM *r, uint32_t carry, uint32_t neg)
+{
+	int i;
+	uint32_t ones = ~0;
+	uint32_t highbit = (!carry && neg) || (carry && !neg);
+
+	for (i = 0; i < r->dmax - 1; ++i) {
+		uint32_t accu;
+
+		ones &= BN_DIGIT(r, i);
+		accu = (BN_DIGIT(r, i) >> 1);
+		accu |= (BN_DIGIT(r, i + 1) << (BN_BITS2 - 1));
+		BN_DIGIT(r, i) = accu;
+	}
+	ones &= BN_DIGIT(r, i);
+	BN_DIGIT(r, i) = (BN_DIGIT(r, i) >> 1) |
+		(highbit << (BN_BITS2 - 1));
+
+	if (ones == ~0 && highbit && neg)
+		memset(r->d, 0x00, bn_size(r));    /* -1 >> 1 = 0. */
+}
+
 /* Montgomery c[] += a * b[] / R % N. */
 /* TODO(ngm): constant time. */
 static void bn_mont_mul_add(struct BIGNUM *c, const uint32_t a,
@@ -244,3 +337,151 @@ void bn_mont_modexp(struct BIGNUM *output, const struct BIGNUM *input,
 	dcrypto_memset(acc_buf, 0, sizeof(acc_buf));
 	dcrypto_memset(aR_buf, 0, sizeof(aR_buf));
 }
+
+/* c[] += a * b[] */
+static uint32_t bn_mul_add(struct BIGNUM *c, uint32_t a,
+			const struct BIGNUM *b, uint32_t offset)
+{
+	int i;
+	uint64_t carry = 0;
+
+	for (i = 0; i < b->dmax; i++) {
+		carry += BN_DIGIT(c, offset + i) +
+			(uint64_t) BN_DIGIT(b, i) * a;
+		BN_DIGIT(c, offset + i) = (uint32_t) carry;
+		carry >>= 32;
+	}
+
+	return carry;
+}
+
+/* c[] = a[] * b[] */
+void bn_mul(struct BIGNUM *c, const struct BIGNUM *a, const struct BIGNUM *b)
+{
+	int i;
+	uint32_t carry = 0;
+
+	memset(c->d, 0, bn_size(c));
+	for (i = 0; i < a->dmax; i++) {
+		BN_DIGIT(c, i + b->dmax - 1) = carry;
+		carry = bn_mul_add(c, BN_DIGIT(a, i), b, i);
+	}
+
+	BN_DIGIT(c, i + b->dmax - 1) = carry;
+}
+
+#define bn_is_even(b) !bn_is_bit_set((b), 0)
+#define bn_is_odd(b) bn_is_bit_set((b), 0)
+
+static int bn_is_zero(const struct BIGNUM *a)
+{
+	int i, result = 0;
+
+	for (i = 0; i < a->dmax; ++i)
+		result |= BN_DIGIT(a, i);
+	return !result;
+}
+
+/* d = (e ^ -1) mod MOD  */
+/* TODO(ngm): this method is used in place of division to calculate
+ * q = N/p, i.e.  q = p^-1 mod (N-1).  The buffer e may be
+ * resized to uint32_t once division is implemented. */
+int bn_modinv_vartime(struct BIGNUM *d, const struct BIGNUM *e,
+		const struct BIGNUM *MOD)
+{
+	/* Buffers for B, D, and U must be as large as e. */
+	uint32_t A_buf[RSA_MAX_WORDS];
+	uint32_t B_buf[RSA_MAX_WORDS / 2];
+	uint32_t C_buf[RSA_MAX_WORDS];
+	uint32_t D_buf[RSA_MAX_WORDS / 2];
+	uint32_t U_buf[RSA_MAX_WORDS / 2];
+	uint32_t V_buf[RSA_MAX_WORDS];
+	int a_neg = 0;
+	int b_neg = 0;
+	int c_neg = 0;
+	int d_neg = 0;
+	int carry1;
+	int carry2;
+	int i = 0;
+
+	struct BIGNUM A;
+	struct BIGNUM B;
+	struct BIGNUM C;
+	struct BIGNUM D;
+	struct BIGNUM U;
+	struct BIGNUM V;
+
+	if (bn_size(e) > sizeof(U_buf))
+		return 0;
+
+	bn_init(&A, A_buf, bn_size(MOD));
+	BN_DIGIT(&A, 0) = 1;
+	bn_init(&B, B_buf, bn_size(MOD) / 2);
+	bn_init(&C, C_buf, bn_size(MOD));
+	bn_init(&D, D_buf, bn_size(MOD) / 2);
+	BN_DIGIT(&D, 0) = 1;
+
+	bn_init(&U, U_buf, bn_size(e));
+	memcpy(U_buf, e->d, bn_size(e));
+
+	bn_init(&V, V_buf, bn_size(MOD));
+	memcpy(V_buf, MOD->d, bn_size(MOD));
+
+	/* Binary extended GCD, as per Handbook of Applied
+	 * Cryptography, 14.61. */
+	for (i = 0;; i++) {
+		carry1 = 0;
+		carry2 = 0;
+		if (bn_is_even(&U)) {
+			bn_rshift(&U, 0, 0);
+			if (bn_is_odd(&A) || bn_is_odd(&B)) {
+				carry1 = bn_signed_add(&A, &a_neg, MOD, 0);
+				carry2 = bn_signed_sub(&B, &b_neg, e, 0);
+			}
+			bn_rshift(&A, carry1, a_neg);
+			bn_rshift(&B, carry2, b_neg);
+		} else if (bn_is_even(&V)) {
+			bn_rshift(&V, 0, 0);
+			if (bn_is_odd(&C) || bn_is_odd(&D)) {
+				carry1 = bn_signed_add(&C, &c_neg, MOD, 0);
+				carry2 = bn_signed_sub(&D, &d_neg, e, 0);
+			}
+			bn_rshift(&C, carry1, c_neg);
+			bn_rshift(&D, carry2, d_neg);
+		} else {  /* U, V both odd. */
+			if (bn_gte(&U, &V)) {
+				assert(!bn_sub(&U, &V));
+				if (bn_signed_sub(&A, &a_neg, &C, c_neg))
+					bn_signed_add(&A, &a_neg, MOD, 0);
+				if (bn_signed_sub(&B, &b_neg, &D, d_neg))
+					bn_signed_add(&B, &b_neg, MOD, 0);
+				if (bn_is_zero(&U))
+					break;  /* done. */
+			} else {
+				assert(!bn_sub(&V, &U));
+				if (bn_signed_sub(&C, &c_neg, &A, a_neg))
+					bn_signed_add(&C, &c_neg, MOD, 0);
+				if (bn_signed_sub(&D, &d_neg, &B, b_neg))
+					bn_signed_add(&D, &d_neg, MOD, 0);
+			}
+		}
+		if ((i + 1) % 1000 == 0)
+			/* TODO(ngm): Poke the watchdog (only
+			 * necessary for q = N/p).  Remove once
+			 * division is implemented. */
+			watchdog_reload();
+	}
+
+	BN_DIGIT(&V, 0) ^= 0x01;
+	if (bn_is_zero(&V)) {
+		while (c_neg)
+			bn_signed_add(&C, &c_neg, MOD, 0);
+		while (bn_gte(&C, MOD))
+			bn_sub(&C, MOD);
+
+		memcpy(d->d, C.d, bn_size(d));
+		return 1;
+	} else {
+		return 0;  /* Inverse not found. */
+	}
+}
diff --git a/chip/g/dcrypto/dcrypto.h b/chip/g/dcrypto/dcrypto.h
index 2147400550..aec3292dae 100644
--- a/chip/g/dcrypto/dcrypto.h
+++ b/chip/g/dcrypto/dcrypto.h
@@ -80,6 +80,11 @@ const uint8_t *DCRYPTO_SHA256_hash(const uint8_t *data, uint32_t n,
 				uint8_t *digest);
 
 /*
+ * BIGNUM utility methods.
+ */
+void DCRYPTO_bn_wrap(struct BIGNUM *b, void *buf, size_t len);
+
+/*
  *  RSA.
  */
 
@@ -125,6 +130,10 @@ int DCRYPTO_rsa_verify(struct RSA *rsa, const uint8_t *digest,
 		const uint32_t sig_len,	enum padding_mode padding,
 		enum hashing_mode hashing);
 
+/* Calculate n = p * q, d = e ^ -1 mod phi. */
+int DCRYPTO_rsa_key_compute(struct BIGNUM *N, struct BIGNUM *d,
+			struct BIGNUM *p, struct BIGNUM *q, uint32_t e);
+
 /*
  *  EC.
  */
diff --git a/chip/g/dcrypto/internal.h b/chip/g/dcrypto/internal.h
index 09685c3004..a79e3b1160 100644
--- a/chip/g/dcrypto/internal.h
+++ b/chip/g/dcrypto/internal.h
@@ -115,6 +115,11 @@ void bn_init(struct BIGNUM *bn, void *buf, size_t len);
 int bn_check_topbit(const struct BIGNUM *N);
 void bn_mont_modexp(struct BIGNUM *output, const struct BIGNUM *input,
 		const struct BIGNUM *exp, const struct BIGNUM *N);
+uint32_t bn_add(struct BIGNUM *c, const struct BIGNUM *a);
+uint32_t bn_sub(struct BIGNUM *c, const struct BIGNUM *a);
+void bn_mul(struct BIGNUM *c, const struct BIGNUM *a, const struct BIGNUM *b);
+int bn_modinv_vartime(struct BIGNUM *r, const struct BIGNUM *e,
+		const struct BIGNUM *MOD);
 
 /*
  * EC.
diff --git a/chip/g/dcrypto/rsa.c b/chip/g/dcrypto/rsa.c
index ff42e00457..7742cea18e 100644
--- a/chip/g/dcrypto/rsa.c
+++ b/chip/g/dcrypto/rsa.c
@@ -625,3 +625,38 @@ int DCRYPTO_rsa_verify(struct RSA *rsa, const uint8_t *digest,
 	dcrypto_memset(signature_buf, 0, sizeof(signature_buf));
 	return ret;
 }
+
+int DCRYPTO_rsa_key_compute(struct BIGNUM *N, struct BIGNUM *d,
+			struct BIGNUM *p, struct BIGNUM *q, uint32_t e_buf)
+{
+	uint32_t ONE_buf = 1;
+	uint32_t phi_buf[RSA_MAX_WORDS];
+	uint32_t q_buf[RSA_MAX_WORDS / 2];
+
+	struct BIGNUM ONE;
+	struct BIGNUM e;
+	struct BIGNUM phi;
+	struct BIGNUM q_local;
+
+	DCRYPTO_bn_wrap(&ONE, &ONE_buf, sizeof(ONE_buf));
+	DCRYPTO_bn_wrap(&phi, phi_buf, bn_size(N));
+	if (!q) {
+		/* q not provided, calculate it. */
+		memcpy(phi_buf, N->d, bn_size(N));
+		bn_init(&q_local, q_buf, bn_size(p));
+		bn_sub(&phi, &ONE);
+		if (!bn_modinv_vartime(&q_local, p, &phi))
+			return 0;
+		q = &q_local;
+		bn_add(&phi, &ONE);
+	} else {
+		bn_mul(N, p, q);
+		memcpy(phi_buf, N->d, bn_size(N));
+	}
+
+	bn_sub(&phi, p);
+	bn_sub(&phi, q);
+	bn_add(&phi, &ONE);
+	DCRYPTO_bn_wrap(&e, &e_buf, sizeof(e_buf));
+	return bn_modinv_vartime(d, &e, &phi);
+}
diff --git a/test/tpm_test/rsa_test.py b/test/tpm_test/rsa_test.py
index 2a751d3ee7..14982d5380 100644
--- a/test/tpm_test/rsa_test.py
+++ b/test/tpm_test/rsa_test.py
@@ -17,7 +17,8 @@ _RSA_OPCODES = {
   'DECRYPT': 0x01,
   'SIGN': 0x02,
   'VERIFY': 0x03,
-  'KEYGEN': 0x04
+  'KEYGEN': 0x04,
+  'KEYTEST': 0x05,
 }
 
 
@@ -102,6 +103,13 @@ def _verify_cmd(padding, hashing, key_len, sig, msg):
                                 ml=struct.pack('>H', sig_len), msg=sig,
                                 dl=struct.pack('>H', digest_len), dig=digest)
 
+def _keytest_cmd(key_len):
+  op = _RSA_OPCODES['KEYTEST']
+  return _RSA_CMD_FORMAT.format(o=op, p=0, h=_HASH['NONE'],
+                                kl=struct.pack('>H', key_len),
+                                ml=struct.pack('>H', 0), msg='',
+                                dl='', dig='')
+
 
 #
 # TEST VECTORS.
@@ -122,6 +130,10 @@ _SIGN_INPUTS = (
   ('PKCS1-PSS', 'SHA256', 768),
 )
 
+_KEYTEST_INPUTS = (
+  (768,),
+  (2048,),
+)
 
 def _encrypt_tests(tpm):
   msg = 'Hello CR50!'
@@ -172,6 +184,21 @@ def _sign_tests(tpm):
     print('%sSUCCESS: %s' % (utils.cursor_back(), test_name))
 
 
+def _keytest_tests(tpm):
+  for data in _KEYTEST_INPUTS:
+    key_len, = data
+    test_name = 'RSA-KEYTEST:%d' % data
+    cmd = _keytest_cmd(key_len)
+    wrapped_response = tpm.command(tpm.wrap_ext_command(subcmd.RSA, cmd))
+    valid = tpm.unwrap_ext_response(subcmd.RSA, wrapped_response)
+    expected = '\x01'
+    if valid != expected:
+      raise subcmd.TpmTestError('%s error:%s%s' % (
+          test_name, utils.hex_dump(valid), utils.hex_dump(expected)))
+    print('%sSUCCESS: %s' % (utils.cursor_back(), test_name))
+
+
 def rsa_test(tpm):
   _encrypt_tests(tpm)
   _sign_tests(tpm)
+  _keytest_tests(tpm)