SHA-1 hashing concept in Git - Step-by-Step Execution

Process Flow - SHA-1 hashing concept

Input Data

↓

Preprocessing

↓

Split into Blocks

↓

Initialize Hash Values

↓

Process Each Block

↓

Compression Function

↓

Update Hash

↓

Final SHA-1 Hash Output

SHA-1 takes input data, processes it in blocks through a compression function, and produces a fixed 40-character hash string.

Execution Sample

Git

echo -n "hello" | sha1sum
2aae6c35c94fcfb415dbe95f408b9ce91ee846ed  -

This command computes the SHA-1 hash of the string "hello" and outputs the hash.

Process Table

Step	Action	Input/State	Output/State
1	Input Data	"hello"	"hello"
2	Preprocessing	"hello"	"hello" + padding + length info
3	Split into Blocks	Preprocessed data	One 512-bit block
4	Initialize Hash Values	N/A	5 initial 32-bit words (h0-h4)
5	Process Block	512-bit block + h0-h4	80 words prepared for rounds
6	Compression Function	80 words + h0-h4	Updated h0-h4 values
7	Update Hash	Updated h0-h4	Final 160-bit hash
8	Output Hash	160-bit hash	"2aae6c35c94fcfb415dbe95f408b9ce91ee846ed" (hex)

💡 All blocks processed; final 160-bit SHA-1 hash produced as hex string.

Status Tracker

Variable	Start	After Step 4	After Step 6	Final
Input Data	"hello"	"hello"	"hello" + padding	"hello" + padding
Blocks	N/A	N/A	One 512-bit block	One 512-bit block
h0-h4	N/A	Initial constants	Updated by compression	Final hash words
Hash Output	N/A	N/A	N/A	"2aae6c35c94fcfb415dbe95f408b9ce91ee846ed"

Key Moments - 3 Insights

Why does the input data get padded before processing?

Padding ensures the data length is a multiple of 512 bits, which is required for SHA-1 to process data in fixed-size blocks, as shown in step 2 and 3 of the execution table.

What are h0 to h4 in the SHA-1 process?

They are initial fixed 32-bit words used as starting hash values, updated during processing (steps 4 to 6), eventually forming the final hash output.

Why is the final output a 40-character string?

Because SHA-1 produces a 160-bit (20-byte) hash, which is represented as 40 hexadecimal characters (2 hex chars per byte), as shown in step 8.

Visual Quiz - 3 Questions

Look at the execution table, what is the state of h0-h4 after step 4?

A Initial constants ready for processing

B Final hash values

C Input data padded

D Blocks split into 512-bit chunks

💡 Hint

Refer to step 4 in the execution table where hash values are initialized.

At which step does the input data get split into blocks?

A Step 5

B Step 2

C Step 3

D Step 7

💡 Hint

Check the 'Split into Blocks' action in the execution table.

If the input string changes, which part of the execution table will definitely change?

A Initialize Hash Values

B Compression Function output

C Padding step only

D Final output hash remains same

💡 Hint

Look at how the compression function output depends on input data blocks and hash values.

Concept Snapshot

SHA-1 hashing:
- Input data is padded to multiple of 512 bits
- Data split into 512-bit blocks
- Initial hash values (h0-h4) set
- Each block processed with compression function
- Final output is 160-bit hash (40 hex chars)
- Used in git to identify objects uniquely

Full Transcript

SHA-1 hashing takes any input data and processes it to produce a fixed-length 40-character hexadecimal string. The process starts by padding the input so its length is a multiple of 512 bits. Then the data is split into 512-bit blocks. Five initial hash values (h0 to h4) are set as constants. Each block is processed through a compression function that updates these hash values. After all blocks are processed, the final 160-bit hash is output as a 40-character hex string. This hash uniquely represents the input data and is used in git to track files and commits.

Practice

1. What does the SHA-1 hash in Git uniquely identify?

easy

A. A specific commit or file content

B. The username of the committer

C. The branch name

D. The date and time of the commit

Solution

1. Step 1: Understand SHA-1 purpose in Git

   SHA-1 generates a unique code based on the content of files or commits.

2. Step 2: Identify what SHA-1 represents

   It uniquely identifies the exact content or commit, not metadata like username or branch.

3. Final Answer:

   A specific commit or file content -> Option A

4. Quick Check:

   SHA-1 = unique content ID [OK]

Hint: SHA-1 hashes content, not user or branch info [OK]

Common Mistakes:

• Thinking SHA-1 identifies user info
• Confusing SHA-1 with branch names
• Assuming SHA-1 stores commit date

2. Which command correctly shows the SHA-1 hash of the latest commit in a Git repository?

easy

A. git log -1 --format=%H

B. git sha1sum HEAD

C. git show --hash

D. git commit --hash

Solution

1. Step 1: Recall Git command to show commit hash

   The command 'git log -1 --format=%H' prints the full SHA-1 hash of the latest commit.

2. Step 2: Check other options for correctness

   The other options are invalid or do not exist in Git syntax.

3. Final Answer:

   git log -1 --format=%H -> Option A

4. Quick Check:

   Correct Git syntax for latest commit SHA-1 is git log -1 --format=%H [OK]

Hint: Use git log with format %H for full commit hash [OK]

Common Mistakes:

• Using non-existent git commands
• Confusing commit message commands with hash commands
• Trying to use sha1sum directly on HEAD

3. Given the command git hash-object README.md, what is the expected output?

medium

A. An error because README.md is not a commit

B. The size of README.md in bytes

C. The last commit hash that modified README.md

D. The SHA-1 hash of the README.md file content

Solution

1. Step 1: Understand git hash-object command

   This command computes and outputs the SHA-1 hash of the file content given (README.md here).

2. Step 2: Verify output meaning

   It does not show file size, commit hashes, or errors for valid files.

3. Final Answer:

   The SHA-1 hash of the README.md file content -> Option D

4. Quick Check:

   git hash-object outputs file content SHA-1 [OK]

Hint: git hash-object hashes file content, not commits [OK]

Common Mistakes:

• Expecting commit hash instead of file hash
• Thinking it outputs file size
• Assuming it errors on normal files

4. You run git hash-object -w file.txt but get an error. What is the most likely cause?

medium

A. The -w option is not valid for git hash-object

B. The file.txt does not exist in the current directory

C. You need to commit before hashing

D. The file.txt is already tracked by Git

Solution

1. Step 1: Understand git hash-object -w usage

   The -w option writes the object to Git's object database; it requires the file to exist.

2. Step 2: Identify cause of error

   If file.txt does not exist, the command errors. Other options are incorrect because -w is valid, committing is not required, and tracking status doesn't cause this error.

3. Final Answer:

   The file.txt does not exist in the current directory -> Option B

4. Quick Check:

   Missing file causes git hash-object -w error [OK]

Hint: Check file existence before git hash-object -w [OK]

Common Mistakes:

• Thinking -w is invalid option
• Assuming commit is required before hashing
• Believing tracked files cause this error

5. You want to verify if a file's content matches a specific Git object SHA-1 hash. Which sequence of commands correctly helps you do this?

hard

A. Run git show SHA1_HASH and compare with file.txt content manually

B. Run git diff SHA1_HASH file.txt to see differences

C. Run git hash-object file.txt and compare output with the SHA-1 hash

D. Run git cat-file -p file.txt to get the hash

Solution

1. Step 1: Understand how to get file's SHA-1 hash

   Use 'git hash-object file.txt' to compute the SHA-1 hash of the file content.

2. Step 2: Compare computed hash with given SHA-1

   If the hashes match, the file content matches the Git object.

3. Final Answer:

   Run git hash-object file.txt and compare output with the SHA-1 hash -> Option C

4. Quick Check:

   Compare file hash with SHA-1 using git hash-object [OK]

Hint: Use git hash-object to get file hash for comparison [OK]

Common Mistakes:

• Using git show to compare file content directly
• Trying git diff with SHA-1 hash and file
• Using git cat-file incorrectly on files