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Diffie-Hellman Version 3 Private Key BLOBs

When a Diffie-Hellman version 3 private key BLOB is exported, it is in a format as follows:


BLOBHEADER        blobheader;
DHPRIVKEY_VER3   dhprivkeyver3;
BYTE p[dhprivkeyver3.bitlenP/8]; 
            // Where P is the prime modulus
BYTE q[dhprivkeyver3.bitlenQ/8]; 
            // Where Q is a large factor of P-1
BYTE g[dhprivkeyver3.bitlenP/8]; 
            // Where G is the generator parameter
BYTE j[dhprivkeyver3.bitlenJ/8]; 
            // Where J is (P-1)/Q
BYTE y[dhprivkeyver3.bitlenP/8]; 
            // Where Y is (G^X) mod P
BYTE x[dhprivkeyver3.bitlenX/8]; 
            // Where X is the private exponent

This BLOB format is exported when the CRYPT_BLOB_VER3 flag is used with CryptExportKey. Because the version is in the BLOB, there is no need to specify a flag when using this BLOB with CryptImportKey.

The following table describes each component of the key BLOB.

FieldDescription
blobheaderA BLOBHEADER structure.
dhprivkeyver3A DHPRIVKEY_VER3 structure. The magic member should be set to 0x34484400 for private keys. Notice that the hexadecimal value is just an ASCII encoding of "DH4".
PThe P value is located directly after the DHPRIVKEY_VER3 structure, and should always be the length in bytes of the DHPRIVKEY_VER3 bitlenP field (bit length of P) divided by eight (little-endian format).
QThe Q value is located directly after the P value and should always be the length in bytes of the DHPRIVKEY_VER3 bitlenQ field divided by eight (little-endian format). If the bitlenQ value is 0, then the value is absent from the BLOB.
GThe G value is located directly after the Q value and should always be the length in bytes of the DHPRIVKEY_VER3 bitlenP field (bit length of P) divided by eight. If the length of the data is one or more bytes shorter than P divided by 8, the data must be padded with the necessary bytes (of zero value) to make the data the desired length (little-endian format).
JThe J value is located directly after the G value and should always be the length in bytes of the DHPRIVKEY_VER3 bitlenJ field divided by eight (little-endian format). If the bitlenJ value is 0, then the value is absent from the BLOB.
YThe Y value, (G^X) mod P, is located directly after the J value, and should always be the length in bytes of the DHPRIVKEY_VER3 bitlenP field (bit length of P) divided by eight. If the length of the data that results from the calculation of (G^X) mod P is one or more bytes shorter than P divided by 8, the data must be padded with the necessary bytes (of zero value) to make the data the desired length (little-endian format).
XThe X value is a random large integer such that the public portion of the DH key pair, Y, is equal to:

Y = (G^X) mod P

 

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

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

 

 

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