Add EVP_CIPHER API for XAES-256-GCM with Key Commitment #2826
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| Original file line number | Diff line number | Diff line change |
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@@ -353,6 +353,15 @@ typedef struct { | |
| uint8_t k1[AES_BLOCK_SIZE]; | ||
| } XAES_256_GCM_CTX; | ||
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| #define XAES_256_GCM_KEY_COMMIT_SIZE (AES_BLOCK_SIZE * 2) | ||
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| typedef struct { | ||
| EVP_AES_GCM_CTX aes_gcm_ctx; | ||
| AES_KEY xaes_key; | ||
| uint8_t k1[AES_BLOCK_SIZE]; | ||
| uint8_t kc[XAES_256_GCM_KEY_COMMIT_SIZE]; | ||
| } XAES_256_GCM_KC_CTX; | ||
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| static EVP_AES_GCM_CTX *aes_gcm_from_cipher_ctx(EVP_CIPHER_CTX *ctx) { | ||
| OPENSSL_STATIC_ASSERT( | ||
| alignof(EVP_AES_GCM_CTX) <= 16, | ||
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@@ -381,13 +390,12 @@ static EVP_AES_GCM_CTX *aes_gcm_from_cipher_ctx(EVP_CIPHER_CTX *ctx) { | |
| case NID_xaes_256_gcm: | ||
| assert(ctx->cipher->ctx_size == sizeof(XAES_256_GCM_CTX)); | ||
| ptr = ctx->cipher_data; | ||
|
Contributor
There was a problem hiding this comment. In my previous comment, I meant keeping this alignment in
Author
There was a problem hiding this comment. Yeah. Since |
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| return &((XAES_256_GCM_CTX *)ptr)->aes_gcm_ctx; | ||
| // XAES-256-GCM-KC | ||
| case NID_xaes_256_gcm_kc: | ||
| assert(ctx->cipher->ctx_size == sizeof(XAES_256_GCM_KC_CTX)); | ||
| ptr = ctx->cipher_data; | ||
| return &((XAES_256_GCM_KC_CTX *)ptr)->aes_gcm_ctx; | ||
| default: | ||
| break; | ||
| } | ||
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@@ -1781,7 +1789,6 @@ Extension to support nonce size less than 24 bytes: | |
| https://eprint.iacr.org/2025/758.pdf#page=24 | ||
| -----------------------------------------------------------------------*/ | ||
| #define XAES_256_GCM_KEY_LENGTH (AES_BLOCK_SIZE * 2) | ||
| #define XAES_256_GCM_MAX_NONCE_SIZE (AES_GCM_NONCE_LENGTH * 2) | ||
| #define XAES_256_GCM_MIN_NONCE_SIZE (20) | ||
| /* | ||
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@@ -1834,15 +1841,7 @@ static int xaes_256_gcm_CMAC_derive_key(AES_KEY *xaes_key, uint8_t *k1, | |
| } | ||
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| static XAES_256_GCM_CTX *xaes_256_gcm_from_cipher_ctx(EVP_CIPHER_CTX *ctx) { | ||
| return (XAES_256_GCM_CTX *)ctx->cipher_data; | ||
| } | ||
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| static int xaes_256_gcm_set_gcm_key(EVP_CIPHER_CTX *ctx, const uint8_t *nonce, int enc) { | ||
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@@ -2033,3 +2032,128 @@ DEFINE_METHOD_FUNCTION(EVP_AEAD, EVP_aead_xaes_256_gcm) { | |
| out->seal_scatter = aead_xaes_256_gcm_seal_scatter; | ||
| out->open_gather = aead_xaes_256_gcm_open_gather; | ||
| } | ||
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| // ------------------------------------------------------------------------------ | ||
| // ---------------- EVP_CIPHER XAES-256-GCM With Key Commitment ----------------- | ||
| // ----------- Reference: https://eprint.iacr.org/2025/758.pdf#page=6 ----------- | ||
| // ------------------------------------------------------------------------------ | ||
| static XAES_256_GCM_KC_CTX *xaes_256_gcm_kc_from_cipher_ctx(EVP_CIPHER_CTX *ctx) { | ||
| return (XAES_256_GCM_KC_CTX *)ctx->cipher_data; | ||
| } | ||
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| static int xaes_256_gcm_extract_key_commitment(AES_KEY *xaes_key, uint8_t *k1, | ||
| uint8_t *key_commitment, const uint8_t* nonce, const unsigned nonce_len) { | ||
| uint8_t W[AES_BLOCK_SIZE]; | ||
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| uint8_t kc_prefix[4] = {0x58, 0x43, 0x4D, 0x54}; | ||
| uint8_t X1[AES_BLOCK_SIZE]; | ||
| OPENSSL_memcpy(W, kc_prefix, 4); | ||
| OPENSSL_memcpy(W + 4, nonce, AES_GCM_NONCE_LENGTH); | ||
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| AES_encrypt(W, X1, xaes_key); | ||
| // For nonce size < 24 | ||
| // Reference: https://eprint.iacr.org/2025/758.pdf#page=24 | ||
| OPENSSL_memcpy(W, nonce + nonce_len - AES_GCM_NONCE_LENGTH, AES_GCM_NONCE_LENGTH); | ||
| W[AES_BLOCK_SIZE-4] = XAES_256_GCM_MAX_NONCE_SIZE - nonce_len; | ||
| W[AES_BLOCK_SIZE-3] = 0x01; | ||
| W[AES_BLOCK_SIZE-2] = 0x00; | ||
| W[AES_BLOCK_SIZE-1] = 0x01; | ||
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| for (size_t i = 0; i < AES_BLOCK_SIZE; i++) { | ||
| W[i] ^= X1[i] ^ k1[i]; | ||
| } | ||
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| AES_encrypt(W, key_commitment, xaes_key); | ||
| /* Since W1[i] and W2[i] are the same except at i = 15, where: | ||
| * W1[15] = x1[15] ^ k1[15] ^ 0x01, and W2[15] = x1[15] ^ k1[15] ^ 0x02, we have: | ||
| * W2[15] = W1[15] ^ 0x03 = (x1[15] ^ k1[15] ^ 0x01) ^ (0x01 ^ 0x02) = W2[15] */ | ||
| W[AES_BLOCK_SIZE-1] ^= 0x03; | ||
| AES_encrypt(W, key_commitment + AES_BLOCK_SIZE, xaes_key); | ||
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| return 1; | ||
| } | ||
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| static int xaes_256_gcm_kc_init(EVP_CIPHER_CTX *ctx, const uint8_t *key, | ||
| const uint8_t *iv, int enc) { | ||
| // Key length: 32 bytes | ||
| if (ctx->key_len != XAES_256_GCM_KEY_LENGTH) { | ||
| OPENSSL_PUT_ERROR(CIPHER, CIPHER_R_BAD_KEY_LENGTH); | ||
| return 0; | ||
| } | ||
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| XAES_256_GCM_KC_CTX *xaes_ctx = xaes_256_gcm_kc_from_cipher_ctx(ctx); | ||
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| // When main key is provided, initialize the xaes-256-gcm context | ||
| if(key != NULL) { | ||
| xaes_256_gcm_ctx_init(&xaes_ctx->xaes_key, xaes_ctx->k1, key); | ||
| } | ||
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| // If iv is provided, even if main key is not, derive a subkey and its key commitment | ||
| /* Note: Unlike the MAC tag, key commitment only depends on the main key | ||
| * and iv, not on ciphertext and aad, therefore we extract it in init when | ||
| * deriving subkey instead of at the end of encrytion like the MAC tag */ | ||
| if(iv != NULL) { | ||
| // Derive subkey | ||
| xaes_256_gcm_set_gcm_key(ctx, iv, enc); | ||
| // Extract key commitment | ||
| EVP_AES_GCM_CTX *gctx = &xaes_ctx->aes_gcm_ctx; | ||
| xaes_256_gcm_extract_key_commitment(&xaes_ctx->xaes_key, xaes_ctx->k1, xaes_ctx->kc, iv, gctx->ivlen); | ||
| } | ||
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| return 1; | ||
| } | ||
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| static int xaes_256_gcm_kc_ctrl(EVP_CIPHER_CTX *ctx, int type, int arg, void *ptr) { | ||
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| XAES_256_GCM_KC_CTX *xaes_ctx = xaes_256_gcm_kc_from_cipher_ctx(ctx); | ||
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| switch(type) { | ||
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Contributor
There was a problem hiding this comment. I think it's more prudent to check that ptr is not NULL. aws-lc/crypto/fipsmodule/cipher/cipher.c Line 185 in 88aec89
Author
There was a problem hiding this comment. Yes. I'll add checking ptr for two new added ctrl commands. |
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| case EVP_CTRL_AEAD_GET_KC: | ||
| if(arg < XAES_256_GCM_KEY_COMMIT_SIZE) { | ||
| OPENSSL_PUT_ERROR(CIPHER, CIPHER_R_BUFFER_TOO_SMALL); | ||
| return 0; | ||
| } | ||
| if(!ptr) { | ||
| OPENSSL_PUT_ERROR(CIPHER, CIPHER_R_BUFFER_INVALID); | ||
| return 0; | ||
| } | ||
| OPENSSL_memcpy(ptr, xaes_ctx->kc, arg); | ||
| return 1; | ||
| /* Since the verification of key commitment (KC) can happen at any time, | ||
| * not only at the end of decryption like for MAC tag, we create a control | ||
| * command for KC verification, rather than a command to set KC, then | ||
| * verifying it at the end of decryption as for checking the MAC tag */ | ||
| case EVP_CTRL_AEAD_VERIFY_KC: | ||
| if(arg < XAES_256_GCM_KEY_COMMIT_SIZE) { | ||
| OPENSSL_PUT_ERROR(CIPHER, CIPHER_R_BUFFER_TOO_SMALL); | ||
| return 0; | ||
| } | ||
| if(!ptr) { | ||
| OPENSSL_PUT_ERROR(CIPHER, CIPHER_R_BUFFER_INVALID); | ||
| return 0; | ||
| } | ||
| if(OPENSSL_memcmp(xaes_ctx->kc, ptr, XAES_256_GCM_KEY_COMMIT_SIZE)) { | ||
| OPENSSL_PUT_ERROR(CIPHER, CIPHER_R_KEY_COMMITMENT_INVALID); | ||
| return 0; | ||
| } | ||
| return 1; | ||
| default: | ||
| return aes_gcm_ctrl(ctx, type, arg, ptr); | ||
| } | ||
| } | ||
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| DEFINE_METHOD_FUNCTION(EVP_CIPHER, EVP_xaes_256_gcm_kc) { | ||
| OPENSSL_memset(out, 0, sizeof(EVP_CIPHER)); | ||
| out->nid = NID_xaes_256_gcm_kc; | ||
| out->block_size = 1; | ||
| out->key_len = XAES_256_GCM_KEY_LENGTH; | ||
| out->iv_len = XAES_256_GCM_MAX_NONCE_SIZE; | ||
| out->ctx_size = sizeof(XAES_256_GCM_KC_CTX); | ||
| out->flags = EVP_CIPH_GCM_MODE | EVP_CIPH_CUSTOM_IV | EVP_CIPH_CUSTOM_COPY | | ||
| EVP_CIPH_FLAG_CUSTOM_CIPHER | EVP_CIPH_ALWAYS_CALL_INIT | | ||
| EVP_CIPH_CTRL_INIT | EVP_CIPH_FLAG_AEAD_CIPHER | EVP_CIPH_FLAG_KC_CIPHER; | ||
| out->init = xaes_256_gcm_kc_init; | ||
| out->cipher = aes_gcm_cipher; | ||
| out->cleanup = aes_gcm_cleanup; | ||
| out->ctrl = xaes_256_gcm_kc_ctrl; | ||
| } | ||
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Can the negative tests that were added for XAES (e.g. shorter nonces) be modified to also test XAES-KC?
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I made changes in
cipher_test.ccto test negative cases with invalid key/nonce, and shorter nonce for bothevp-xaes-256-gcmandevp-xaes-256-gcm-kc.