在java中计算git packfile sha1校验和

Question

I am learning about the Git packfile and currently trying to reproduce (in Java) what I believe to be the SHA1 20-byte checksum for the entire packfile. I take the byte array from, and including, the "PACK" 4-byte header to the end of the last packaged object's compressed data. Everything I have read indicates that the next 20 bytes is the SHA1 checksum for the entire packfile.

The 20-byte checksum that is part of the byte array received from Git is: B910248BF9B63AC53595E3835CA57BDAF08DA830

I use the following to calculate my own SHA1 checksum:
crypt = MessageDigest.getInstance("SHA-1");
crypt.reset();
crypt.update(testData);
byte [] result = crypt.digest();

My result ends up as: B910248BF9B63AC53595E3835CA57BDAF08DA813

I am baffled at how only the last byte of my result can be different from Git's (if I am using the correct part of the byte stream). If the only problem was the range of data passed to digest() then the entire calculated checksum would most likely look different.

Any ideas?

Answer 1

I am learning about the Git packfile and currently trying to reproduce (in Java) what I believe to be the SHA1 20-byte checksum for the entire packfile. I take the byte array from, and including, the "PACK" 4-byte header to the end of the last packaged object's compressed data. Everything I have read indicates that the next 20 bytes is the SHA1 checksum for the entire packfile.

The 20-byte checksum that is part of the byte array received from Git is: B910248BF9B63AC53595E3835CA57BDAF08DA830

I use the following to calculate my own SHA1 checksum:
crypt = MessageDigest.getInstance("SHA-1");
crypt.reset();
crypt.update(testData);
byte [] result = crypt.digest();

My result ends up as: B910248BF9B63AC53595E3835CA57BDAF08DA813

I am baffled at how only the last byte of my result can be different from Git's (if I am using the correct part of the byte stream). If the only problem was the range of data passed to digest() then the entire calculated checksum would most likely look different.

Any ideas?

Answer 2

I am learning about the Git packfile and currently trying to reproduce (in Java) what I believe to be the SHA1 20-byte checksum for the entire packfile. I take the byte array from, and including, the "PACK" 4-byte header to the end of the last packaged object's compressed data. Everything I have read indicates that the next 20 bytes is the SHA1 checksum for the entire packfile.

The 20-byte checksum that is part of the byte array received from Git is: B910248BF9B63AC53595E3835CA57BDAF08DA830

I use the following to calculate my own SHA1 checksum:
crypt = MessageDigest.getInstance("SHA-1");
crypt.reset();
crypt.update(testData);
byte [] result = crypt.digest();

My result ends up as: B910248BF9B63AC53595E3835CA57BDAF08DA813

I am baffled at how only the last byte of my result can be different from Git's (if I am using the correct part of the byte stream). If the only problem was the range of data passed to digest() then the entire calculated checksum would most likely look different.

Any ideas?