[DIRTSEC-4] Offset value set to 0 when extracting DBC from hmac-sha1 output

• Type: Bug
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Environment: JavaME mobile phone platform and validation server.
• Assignee: Unassigned
• Reporter: javier tellez
• Created: 22/May/07 19:40
• Updated: 23/May/07 08:29

Description

Hotp.java class implements RFC4226 "HOTP: An HMAC-Based One-Time Password Algorithm".
In that class,

hotp.generate(secret, counter, digits) uses
int offset = 0;

Section 5.4 from RFC
"The following code example describes the extraction of a dynamic
binary code given that hmac_result is a byte array with the HMACSHA-1 result:
int offset = hmac_result[19] & 0xf ;
int bin_code = (hmac_result[offset] & 0x7f) << 24

(hmac_result[offset+1] & 0xff) << 16

(hmac_result[offset+2] & 0xff) << 8

(hmac_result[offset+3] & 0xff) "

that is, the offset is the least significant nibble from the last byte of
hotp.stepOne() output (the 20 bytes from hmac-sha1(k,c))

Solved by setting offset to this value

int offset = hmac_result[19] & 0xf;