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Questions and Answers
What do non-invasive attacks require?
What do non-invasive attacks require?
access to the internal components of the device
What is the goal of non-invasive physical attacks, also known as side-channel attacks?
What is the goal of non-invasive physical attacks, also known as side-channel attacks?
What type of attacks exploit side channels?
What type of attacks exploit side channels?
Physical Attacks
Passive Side-Channel Attacks involve the observer inducing faults.
Passive Side-Channel Attacks involve the observer inducing faults.
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Timing Side Channel in SEH Exercise 2 Code snippet checks whether input complies with __________.
Timing Side Channel in SEH Exercise 2 Code snippet checks whether input complies with __________.
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What happens if the most significant bit (MSBit) of byte 𝒚𝒋,𝒊 is 1?
What happens if the most significant bit (MSBit) of byte 𝒚𝒋,𝒊 is 1?
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What is the result if 𝑦𝑗,𝑖 = 1101 0100₂ is left-shifted and XORed with 1 0001 1011?
What is the result if 𝑦𝑗,𝑖 = 1101 0100₂ is left-shifted and XORed with 1 0001 1011?
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Multiplication in GF(2⁸) takes one cycle longer when the most significant bit (MSBit) is 1. (True/False)
Multiplication in GF(2⁸) takes one cycle longer when the most significant bit (MSBit) is 1. (True/False)
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Match the steps in the beginning of AES encryption with their descriptions:
Match the steps in the beginning of AES encryption with their descriptions:
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What does the evaluation of strcmp() depend on, in the context of matched characters in a sequence?
What does the evaluation of strcmp() depend on, in the context of matched characters in a sequence?
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How many rounds are involved in the AddRoundKey operation in AES?
How many rounds are involved in the AddRoundKey operation in AES?
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In AES, how many bits does the 128-bit plaintext consist of?
In AES, how many bits does the 128-bit plaintext consist of?
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How is each byte of the data block replaced in the SubBytes operation of AES?
How is each byte of the data block replaced in the SubBytes operation of AES?
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How many rounds are involved in the Shift Rows operation in AES?
How many rounds are involved in the Shift Rows operation in AES?
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What kind of field is used for multiplication in the Mix Columns operation in AES?
What kind of field is used for multiplication in the Mix Columns operation in AES?
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What does the multiplication in Galois Field GF(8) involve in AES?
What does the multiplication in Galois Field GF(8) involve in AES?
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Study Notes
Integrated Circuit Packaging
- A semiconductor die is the functional circuit of a microchip
- The die is protected by a passivation layer, which is the target of invasive attacks
- The die is connected to the lead frame through bonding wires, and is encased in a resin mold (chip case)
Physical Attacks
- Physical attacks require physical accessibility to the target system
- Non-invasive attacks do not physically harm the device, but can exploit side channels
- Semi-invasive attacks require access to the internal components of the device, but the passivation layer remains intact
- Invasive attacks require access to the internal components of the device and may involve tampering or partially destroying the passivation layer
Non-Invasive Physical Attacks
- Non-invasive physical attacks are also known as side-channel attacks
- The goal of these attacks is to gather information about cryptographic operations in embedded systems
- The attacker has full control over power and clock lines
- Passive side-channel attacks involve observing physical properties, such as timing, power consumption, electromagnetic fields, and acoustic signals
- Active side-channel attacks involve inducing faults, such as fault analysis and glitch analysis
Passive Side-Channel Attacks
- Side-channel analysis involves observing physical properties to gather information about secret keys
- Side-channel information can be obtained through electromagnetic emissions, heat, timing, and power consumption
Timing Side Channel in SEH Exercise 2
- The code snippet checks whether the input complies with a secret passphrase
- For each correctly matched character, the evaluation of strcmp() takes longer, revealing a timing side channel
Timing Attack on AES
- Timing attacks can be used to compromise the security of AES encryption
- Each byte of the plaintext is encoded with one byte of the key using bitwise XOR
- The AES encryption process involves multiple rounds of substitution and permutation
Advanced Encryption Standard (AES)
- AES is a symmetric-key block cipher that uses a substitution-permutation network
- The encryption process involves 10 rounds of AddRoundKey, SubBytes, ShiftRows, and MixColumns
- Each round involves adding a round key, substituting bytes using a substitution box, shifting rows, and mixing columns
SubBytes
- SubBytes involves replacing each byte of the data block with another byte according to a known rule
- The substitution box is a fixed table that is used to substitute bytes
ShiftRows
- ShiftRows involves shifting bytes row by row
- The bytes are shifted in a circular manner, with the last byte of each row becoming the first byte of the next row
MixColumns
- MixColumns involves mixing bytes within each column
- The mixing is done using a matrix multiplication in a Galois field
Closer Look at Mixed Column
- The mixed column operation involves multiplying bytes with 1, 2, or 3
- The multiplication is performed in a Galois field, using a binary polynomial representation of bytes
- The multiplication takes one cycle longer when the most significant bit (MSBit) is 1
Attack Setting
- The attacker encrypts random data blocks using AES and observes the timing of a single byte operation inside MixColumn
- The attacker can learn the timing difference between MSBit 1 and MSBit 0, which can be used to compromise the security of the AES encryption.### S-Box and Its Function
- S-Box is a lookup table used in substitution-permutation networks
- It takes a plaintext byte
xand a key bytekas inputs, and produces a ciphertext bytey - The S-Box operation is a bitwise XOR of
xandk, followed by a lookup in the table
Basic Principle of Attack
- The attack is based on the fact that the most-significant bit of the S-Box output
yis known - The attacker makes a hypothesis about the key byte
kand computes the corresponding S-Box outputy' - If the most-significant bit of
y'matches the known bit ofy, the key hypothesis could be correct or wrong - If the most-significant bit of
y'does not match, the key hypothesis is wrong
Number of Hypotheses
- There are 2^8 = 256 possible key hypotheses for a single byte
Learning About the Key
- Encrypt multiple plaintexts
pisuch that the bytespi[j]are known - For each known plaintext byte, compute the S-Box output for all 256 possible key hypotheses
- Compare the most-significant bit of each output with the known bit of
y - The key hypothesis that leads to the same bit is the correct key byte
Attack Strategy
- Test with multiple bytes to reduce the number of possible key hypotheses
- Use the known most-significant bit of
yto eliminate wrong key hypotheses
Alternative Attack
- If only the total encryption time is known, the attack can be modified
- Observe the timing of multiple encryptions with random inputs
- Select one S-Box output as the attack target and calculate the average timing for each key hypothesis
- Compare the differences between the average timings and select the key with the largest difference
Attack on AES Software Implementation
- An example attack on an AES software implementation was performed using 10,000 timing measurements
- The attack resulted in the recovery of the key byte
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Description
Learn about non-invasive physical attacks on embedded hardware, including the security of integrated circuit packaging and the role of semiconductor die and bonding wires.
