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src/drivers/sw_crypto: Correct RSA lengths in the example sw crypto backend driver

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Maximum plaintext length was wrong, it is just k - 2 * hLen -2, where k is RSA key modulus length and hLen is hash length

Also minimum block that can be encrypted didn't make sense for RSA-OAEP, it is just 1 byte, the rest will be padded.

Signed-off-by: Jukka Laitinen <jukkax@ssrc.tii.ae>
This commit is contained in:
Jukka Laitinen
2023-03-20 11:22:41 +02:00
committed by Daniel Agar
parent b26669b085
commit 631046b962
1 changed files with 19 additions and 23 deletions
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src/drivers/sw_crypto/crypto.c
+19 -23
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@@ -59,6 +59,10 @@ extern void libtomcrypt_init(void);
#define SECMEM_FREE XFREE
#endif
#define SHA256_HASHLEN 32
#define OAEP_MAX_RSA_MODLEN 256 /* RSA2048 */
#define OAEP_MAX_MSGLEN (OAEP_MAX_RSA_MODLEN - 2 * SHA256_HASHLEN - 2)
/*
* For now, this is just a dummy up/down counter for tracking open/close calls
*/
@@ -382,12 +386,22 @@ bool crypto_get_encrypted_key(crypto_session_handle_t handle,
max_len);
} else {
// The key size, encrypted, is a multiple of minimum block size for the algorithm+key
size_t min_block = crypto_get_min_blocksize(handle, encryption_key_idx);
*max_len = key_sz / min_block * min_block;
switch (handle.algorithm) {
if (key_sz % min_block) {
*max_len += min_block;
case CRYPTO_RSA_OAEP:
/* The length is the RSA key modulus length, and the maximum plaintext
* length is calculated from that. This is now just fixed for RSA2048,
* but one could also parse the RSA key
* (encryption_key_idx) here and calculate the lengths.
*/
*max_len = key_sz <= OAEP_MAX_MSGLEN ? OAEP_MAX_RSA_MODLEN : 0;
ret = true;
break;
default:
*max_len = 0;
break;
}
}
@@ -427,24 +441,6 @@ size_t crypto_get_min_blocksize(crypto_session_handle_t handle, uint8_t key_idx)
ret = 64;
break;
case CRYPTO_RSA_OAEP: {
rsa_key enc_key;
size_t pub_key_sz;
uint8_t *pub_key = (uint8_t *)crypto_get_key_ptr(handle.keystore_handle, key_idx, &pub_key_sz);
initialize_tomcrypt();
if (pub_key &&
rsa_import(pub_key, pub_key_sz, &enc_key) == CRYPT_OK) {
ret = ltc_mp.unsigned_size(enc_key.N);
rsa_free(&enc_key);
} else {
ret = 0;
}
}
break;
default:
ret = 1;
}