Understanding Substitution Cipher and Key Architecture
All information on the blockchain is publicly accessible, allowing participants to view transactions within blocks and create new ones. But does this openness raise concerns about identity impersonation or unauthorized Bitcoin transactions?
As highlighted in our previous column, digital signatures—rooted in cryptographic technology—prevent such tampering. This article delves into the fundamentals of cipher systems.
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Basic Substitution Cipher Explained
A cipher transforms readable information into an unreadable format, ensuring only intended recipients can decode it. In a substitution cipher, each plaintext character is replaced with a corresponding ciphertext character using a predefined method.
Example:
A plaintext "SEND ME MONEY." becomes "EIAVSQISQKAIMF" when substituted via a cipher table (Figure 1). The receiver reverses the process using the same table—the "key"—to restore the original message.
Key Point:
Pre-shared cipher tables act as decryption keys, securing transactions against interception.
Implementing Substitution Ciphers Digitally
Computers use character encoding (e.g., ASCII/Unicode) to represent letters as numbers. By altering these numerical sequences mathematically, plaintext converts to ciphertext.
Formula-Based Encryption:
Using f(x) = ax + b (e.g., a=3, b=6), "SEND ME MONEY." encrypts to "ESRPAOSAOURSWD." The receiver applies the inverse function f^(-1)(x) = (x-b)/a to decrypt.
Security Tip:
Frequent updates to coefficients a and b enhance cipher variability per transmission.
Challenges of Symmetric Key Cryptography
While effective, symmetric (private) key systems face risks:
- Key Exposure: If intercepted during initial exchange, all subsequent communications are compromised.
- Statistical Attacks: Common letter frequencies in languages can aid cipher cracking.
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FAQ Section
Q1: Why is blockchain data public yet secure?
A1: Transparency ensures auditability, while cryptographic signatures prevent tampering.
Q2: How does a substitution cipher differ from modern encryption?
A2: Substitution ciphers replace characters directly, while modern methods (e.g., AES) use complex algorithms and longer keys.
Q3: What’s the main drawback of symmetric keys?
A3: Secure key distribution is challenging; exposed keys compromise entire systems.
Beyond Substitution: Public Key Cryptography
Next, we’ll explore "public key cryptography," which resolves key-exchange vulnerabilities by using paired public/private keys.
About the Author
Hitoshi Ueno, PhD (Engineering), is a professor at Tokyo Ueno Campus, specializing in biosignal processing and software research. Formerly a systems architect at Hitachi, he combines academia with industry expertise.
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