[saiyam-jain-dev]

Describe your change

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• Documentation change?

Ascii85 (Base85) is designed to encode arbitrary binary byte streams. The previous implementation in ciphers/base85.py had several bugs:

1. �scii85_encode called data.decode("utf-8") directly on raw byte inputs. Any binary payload containing bytes $\ge 128$ (e.g. �"\xff\xfe\xfd") crashed with UnicodeDecodeError.
2. �scii85_decode reconstructed bytes by converting 8-bit values to Python chr(val) strings and encoding to UTF-8 (�ytes(result, "utf-8")). This corrupted any decoded byte $\ge 128$ into multi-byte UTF-8 sequences (e.g. byte \x80 turned into \xc2\x80, and \xff turned into \xc3\xbf).
3. Out-of-range character validation and invalid length handling were missing.

This PR:

• Refactors �scii85_encode and �scii85_decode to operate strictly on raw bytes and 32-bit big-endian chunks adhering to Ascii85 (RFC 1924 / Adobe Ascii85 specification).
• Produces identical results to Python standard library �ase64.a85encode and �ase64.a85decode.
• Validates Ascii85 character bounds ( \le \text{ord}(c) \le 117$) and detects invalid single-remainder lengths.
• Adds comprehensive doctests verifying edge cases and validating all 256 single-byte payloads against standard library outputs.

Checklist

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues, then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".