Parse a public key in PEM format error

linhongz commented 7 months ago

Summary

I stored the public key in PEM format inside an array in ROM, then attempted to parse it using mbedtls_pk_parse_public_key, but it failed.

System information

Mbed TLS version: 3.1.0 Operating system and version: ARM Cortex-M4F bare metal Configuration:

#define MBEDTLS_AES_ROM_TABLES
#define MBEDTLS_GENPRIME
#define MBEDTLS_NO_PLATFORM_ENTROPY
#define MBEDTLS_ENTROPY_HARDWARE_ALT
#define MBEDTLS_PKCS1_V15
#define MBEDTLS_PKCS1_V21
#define MBEDTLS_AES_C
#define MBEDTLS_ASN1_PARSE_C
#define MBEDTLS_ASN1_WRITE_C
#define MBEDTLS_BASE64_C
#define MBEDTLS_BIGNUM_C
#define MBEDTLS_CTR_DRBG_C
#define MBEDTLS_ENTROPY_C
#define MBEDTLS_MD_C
#define MBEDTLS_OID_C
#define MBEDTLS_PEM_PARSE_C
#define MBEDTLS_PEM_WRITE_C
#define MBEDTLS_PK_C
#define MBEDTLS_PK_PARSE_C
#define MBEDTLS_PK_WRITE_C
#define MBEDTLS_RSA_C
#define MBEDTLS_SHA224_C
#define MBEDTLS_SHA256_C
#define MBEDTLS_SHA512_C

Compiler and options: ARMCC

Steps to reproduce

The following is my minimal implementation:

const char publicKey[] =    "-----BEGIN RSA PUBLIC KEY-----\n"\
                            "MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEArM6wMjgCViDkO2hBA0tn\n"\
                            "zKn0FWN77tf4DwJGabvddpE2sji4ixX8jsw3mZfvFFwlVLpUYBPDebytNkandIK6\n"\
                            "4q0C5sUa38RiUw8SLyfXF/oOXkJIV6VM4wZGW9kgC7gPq1SfUTP27VQP5ms6TA3K\n"\
                            "F07FAnCp2B8Abv6DuzN4Z6bBvch56JkSKnBtrt3rEDnXvHh1o/i3oGdiZTWfax2Q\n"\
                            "8zDykFr8dTkJI2WvemQ8qV2t9FPdGbuV2Gfh6FrleYRpPuvZnlernMZY3nh7/bYn\n"\
                            "E6zszNWf+6ay9VRvEFeNKbsmmA1duk8VHdGywfFPxSd0ny9cdp5x4wCqhby5cx3E\n"\
                            "gQIDAQAB\n"\
                            "-----END RSA PUBLIC KEY-----\n";
mbedtls_rsa_context *loadRsa;
mbedtls_entropy_context entropy;
mbedtls_ctr_drbg_context ctr_drbg;
mbedtls_pk_context pk;
const char *pers = "rsa_key_generation";

mbedtls_entropy_init(&entropy);
mbedtls_ctr_drbg_init(&ctr_drbg);
mbedtls_pk_init(&pk);
mbedtls_ctr_drbg_seed(&ctr_drbg, mbedtls_entropy_func, &entropy,
                                    (const uint8_t *) pers, strlen(pers));

mbedtls_pk_parse_public_key(&pk, (const unsigned char *)publicKey, strlen((char *)publicKey) + 1);
mbedtls_pk_get_type(&pk);
loadRsa = mbedtls_pk_rsa(pk);

mbedtls_pk_parse_public_key-->>pk_get_rsapubkey-->>

/* Import N */
if( ( ret = mbedtls_asn1_get_tag( p, end, &len, MBEDTLS_ASN1_INTEGER ) ) != 0 )
    return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_INVALID_PUBKEY, ret ) );

Here is where the function ultimately fails to execute. Could it be that the format of the array where I stored the keys is incorrect? The key pair was generated by mbedTLS on the platform I'm using. I also tried using key pairs generated by OpenSSL, but encountered the same issue

Additional information

private key:

gilles-peskine-arm commented 7 months ago

Change BEGIN RSA PUBLIC KEY to BEGIN PUBLIC KEY and same with the END.

There are two popular binary formats for RSA public keys: the PKCS1 format that's specifically about for keys, and the SubjectPublicKeyInfo format that can encode other key types such as ECC. Mbed TLS supports both transparently for DER. For PEM, you need to have the correct BEGIN/END descriptor.

This also happens with RSA private keys (PKCS1 vs PKCS8) and ECC private key (SEC1 vs PKCS8). Transparent for DER parsing, but a PEM file needs to have the correct header.

Did some popular tool produce that PEM encoding with a mismatched header? We could make the Mbed TLS parsing code sloppier, I don't think it would be harmful except for making the code messier, but I don't remember this being requested before.

linhongz commented 7 months ago

Change BEGIN RSA PUBLIC KEY to BEGIN PUBLIC KEY and same with the END.

There are two popular binary formats for RSA public keys: the PKCS1 format that's specifically about for keys, and the SubjectPublicKeyInfo format that can encode other key types such as ECC. Mbed TLS supports both transparently for DER. For PEM, you need to have the correct BEGIN/END descriptor.

This also happens with RSA private keys (PKCS1 vs PKCS8) and ECC private key (SEC1 vs PKCS8). Transparent for DER parsing, but a PEM file needs to have the correct header.

Did some popular tool produce that PEM encoding with a mismatched header? We could make the Mbed TLS parsing code sloppier, I don't think it would be harmful except for making the code messier, but I don't remember this being requested before.

Sorry for the oversight. I manually edited the key header based on what I saw in the code. After changing it to the standard PEM format, importing it worked fine. Additionally, regarding storing PEM format in an array, are there any special requirements for newline characters like '\r', '\n' or spaces?

Mbed-TLS / mbedtls