Hawkynt

Ones-Complement

Internet protocol checksum using one’s complement arithmetic. Sums 16-bit words with end-around carry, then inverts all bits. Used in IP, TCP, UDP, and ICMP headers for packet integrity verification.

Properties

Property	Value
Category	Checksums
Sub-category	Internet Protocol
Security status	🎓 Educational Only
Complexity	Beginner
Inventor	Internet Protocol designers
Year	1974
Origin	🇺🇸 United States
Source	algorithms/checksum/complement.js

Security

Status: 🎓 Educational Only

No vulnerabilities are recorded for this implementation.

Notes

• Algorithm: sum 16-bit words, add carry back (end-around carry), then NOT
• Used in: IPv4, TCP, UDP, ICMP headers
• Verification: sum of data + checksum should be 0xFFFF
• Handles odd-length data by padding with zero byte
• End-around carry: carry bits are added back to sum
• One’s complement: invert all bits (~sum)
• Detects most common errors but not all reordering

Documentation

• RFC 1071 - Computing Internet Checksum
• Internet Checksum on Wikipedia
• TCP/IP Checksum Calculation

References

• Linux kernel Internet checksum implementation

Test vectors

3 vectors ship with this algorithm and run in the test suite. Byte values are hexadecimal.

Vector 1 — Two 16-bit words 0x0001 + 0x0002, complement 0xFFFC (RFC 1071 section 1 definition)

Field	Value
input	00010002
expected	fffc

Vector 2 — IP header fragment

Field	Value
input	0001f203f4f5f6f7
expected	220d

Vector 3 — Single word 0xFFFF, complement 0x0000 (RFC 1071 section 1 definition)

Field	Value
input	ffff
expected	0000

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