Edit

RSA/Schannel Key BLOBs

  • Article

BLOBs are used with the RSA/Schannel provider to export keys from, and import keys into, the cryptographic service provider (CSP).

Public Key BLOBs

Public key BLOBs, type PUBLICKEYBLOB, are used to store public keys. These keys are exported and imported as a sequence of bytes with the following format.

PUBLICKEYSTRUC  publickeystruc ;
RSAPUBKEY       rsapubkey;
BYTE            modulus[rsapubkey.bitlen/8];

The following table describes each public key component. All values are in little-endian format.

Field Description
modulus A BYTE sequence. The public key modulus data is located directly after the RSAPUBKEY structure. The length of this data varies, depending on the length of the public key. The number of bytes can be determined by dividing the value of the bitlen member of RSAPUBKEY by eight.
publickeystruc A PUBLICKEYSTRUC structure.
rsapubkey A RSAPUBKEY structure. The magic member must be set to 0x31415352. This hexadecimal value is the ASCII encoding of RSA1.

 

Note

Public key BLOBs are not encrypted. They contain public keys in plaintext form.

 

Private Key BLOBs

Private key BLOBs, type PRIVATEKEYBLOB, are used to store public/private key pairs. These keys are exported and imported as a sequence of bytes with the following format.

PUBLICKEYSTRUC  publickeystruc ;
RSAPUBKEY       rsapubkey;
BYTE            modulus[rsapubkey.bitlen/8];
BYTE            prime1[rsapubkey.bitlen/16];
BYTE            prime2[rsapubkey.bitlen/16];
BYTE            exponent1[rsapubkey.bitlen/16];
BYTE            exponent2[rsapubkey.bitlen/16];
BYTE            coefficient[rsapubkey.bitlen/16];
BYTE            privateExponent[rsapubkey.bitlen/8];

If the key BLOB is encrypted, then everything but the PUBLICKEYSTRUC portion of the BLOB is encrypted.

Note

The encryption algorithm and encryption key parameters are not stored along with the private key BLOB. It is the responsibility of the application to manage this information.

 

The following table describes each private key BLOB component.

Note

These fields correspond to the fields described in section 7.2 of Public Key Cryptography Standards (PKCS) #1 with minor differences.

 

Field Description
exponent1 A BYTE sequence. The first exponent. This has a numeric value of d mod (p – 1).
exponent2 A BYTE sequence. The second exponent. This has a numeric value of d mod (q – 1).
coefficient A BYTE sequence. Coefficient. This has a numeric value of (inverse of q) mod p.
Modulus A BYTE sequence. The modulus. This has a string that contains Prime1 * Prime2. It is often known as n.
prime1 A BYTE sequence. Prime number 1, often known as p.
prime2 A BYTE sequence. Prime number 2, often known as q.
publickeystruc A PUBLICKEYSTRUC structure.
privateExponent A BYTE sequence. The private exponent, often known as d.
rsapubkey A RSAPUBKEY structure. The magic member must be set to 0x32415352. This hexadecimal value is the ASCII encoding of RSA2.

 

Note

Private key BLOBs are not encrypted. They contain private keys in plaintext form.

 

When calling CryptExportKey, the developer can choose whether to encrypt the key. The PRIVATEKEYBLOB is encrypted if the hExpKey parameter contains a valid handle to a session key. Everything but the PUBLICKEYSTRUC portion of the BLOB is encrypted.

Note

The encryption algorithm and encryption key parameters are not stored along with the private key BLOB. The application must manage and store this information. If zero is passed for hExpKey, the private key will be exported without encryption.

 

Caution

It is dangerous to export private keys without encryption because they are then vulnerable to interception and use by unauthorized entities.

 

Simple Key BLOBs

Simple key BLOBs, type SIMPLEBLOB, are used to store and transport session keys. These are always encrypted with a key exchange public key. These keys are exported and imported as a sequence of bytes with the following format.

PUBLICKEYSTRUC  publickeystruc ;
ALG_ID          algid;
BYTE            encryptedkey[rsapubkey.bitlen/8];

The following table describes each simple BLOB component.

Field Description
algid An ALG_ID structure. This typically specifies the CALG_RSA_KEYX algorithm, which indicates that the session key data was encrypted with a key exchange public key, using the RSA Public Key algorithm.
encryptedkey A BYTE sequence. The encrypted session key data is in the form of a PKCS #1, type 2 encryption block. For information about this data format, see the Public Key Cryptography Standards (PKCS), published by RSA Data Security, Inc.
publickeystruc A PUBLICKEYSTRUC structure.

 

This data is always the same size as the modulus of the public key. For example, public keys generated by the Microsoft Base Cryptographic Provider are always 512 bits (64 bytes) in length, so the encrypted session key data is also always 64 bytes.