diff --git a/backend/src/types/validation/IsBase64Options.ts b/backend/src/types/validation/IsBase64Options.ts
new file mode 100644
index 0000000..e008f27
--- /dev/null
+++ b/backend/src/types/validation/IsBase64Options.ts
@@ -0,0 +1,3 @@
+export interface IsBase64Options {
+    byteLength?: number;
+}
diff --git a/backend/src/types/validation/NullOrIntOptions.ts b/backend/src/types/validation/NullOrIntOptions.ts
new file mode 100644
index 0000000..baded07
--- /dev/null
+++ b/backend/src/types/validation/NullOrIntOptions.ts
@@ -0,0 +1,4 @@
+export type NullOrIntOptions = {
+    min?: number;
+    max?: number;
+};
diff --git a/backend/src/utils/toBase64.spec.ts b/backend/src/utils/toBase64.spec.ts
new file mode 100644
index 0000000..73684bf
--- /dev/null
+++ b/backend/src/utils/toBase64.spec.ts
@@ -0,0 +1,39 @@
+import { toBase64 } from './toBase64';
+
+describe('toBase64', () => {
+    it('encodes utf8 strings', () => {
+        expect(toBase64('hello')).toBe('aGVsbG8=');
+    });
+
+    it('encodes empty strings', () => {
+        expect(toBase64('')).toBe('');
+    });
+
+    it('encodes buffers', () => {
+        const bytes = Buffer.from([0x01, 0x02, 0x03]);
+
+        expect(toBase64(bytes)).toBe(bytes.toString('base64'));
+    });
+
+    it('encodes empty buffers', () => {
+        expect(toBase64(Buffer.alloc(0))).toBe('');
+    });
+
+    it('encodes binary buffers without utf8 interpretation', () => {
+        const bytes = Buffer.from([0xff, 0xfe, 0xfd]);
+
+        expect(toBase64(bytes)).toBe('//79');
+    });
+
+    it('round-trips utf8 strings through base64 decode', () => {
+        const original = 'upload-test-🔒';
+
+        expect(Buffer.from(toBase64(original), 'base64').toString('utf8')).toBe(original);
+    });
+
+    it('round-trips buffers through base64 decode', () => {
+        const original = Buffer.alloc(12, 0xab);
+
+        expect(Buffer.from(toBase64(original), 'base64')).toEqual(original);
+    });
+});
diff --git a/backend/src/utils/toBase64.ts b/backend/src/utils/toBase64.ts
new file mode 100644
index 0000000..697154b
--- /dev/null
+++ b/backend/src/utils/toBase64.ts
@@ -0,0 +1,2 @@
+export const toBase64 = (input: Buffer | string): string =>
+    typeof input === 'string' ? Buffer.from(input, 'utf8').toString('base64') : input.toString('base64');
diff --git a/backend/src/validation/decorators/isBase64.spec.ts b/backend/src/validation/decorators/isBase64.spec.ts
new file mode 100644
index 0000000..cdc6b6b
--- /dev/null
+++ b/backend/src/validation/decorators/isBase64.spec.ts
@@ -0,0 +1,253 @@
+import 'reflect-metadata';
+import { validateSync, type ValidationError } from 'class-validator';
+import { toBase64 } from '../../utils/toBase64';
+import { IsBase64 } from './isBase64';
+
+class UnboundedBase64Dto {
+    @IsBase64()
+    payload: string;
+}
+
+class TwelveByteBase64Dto {
+    @IsBase64({ byteLength: 12 })
+    iv: string;
+}
+
+class ThirtyTwoByteBase64Dto {
+    @IsBase64({ byteLength: 32 })
+    secretKey: string;
+}
+
+class DualFieldDto {
+    @IsBase64({ byteLength: 12 })
+    iv: string;
+
+    @IsBase64()
+    ciphertext: string;
+}
+
+const validateField = <T extends object>(DtoClass: new () => T, property: keyof T & string, value: unknown) => {
+    const dto = Object.assign(new DtoClass(), { [property]: value });
+
+    return validateSync(dto);
+};
+
+const constraintMessage = (errors: ValidationError[], property: string): string | undefined => {
+    const error = errors.find(e => e.property === property);
+
+    return error?.constraints?.isBase64;
+};
+
+const accepts = <T extends object>(DtoClass: new () => T, property: keyof T & string, value: unknown) => {
+    expect(validateField(DtoClass, property, value)).toHaveLength(0);
+};
+
+const rejects = <T extends object>(DtoClass: new () => T, property: keyof T & string, value: unknown) => {
+    expect(validateField(DtoClass, property, value).length).toBeGreaterThan(0);
+};
+
+describe('IsBase64', () => {
+    describe('without byteLength', () => {
+        it('accepts standard base64 for short utf8 payloads', () => {
+            accepts(UnboundedBase64Dto, 'payload', toBase64('hello'));
+        });
+
+        it('accepts base64 without padding when length allows', () => {
+            accepts(UnboundedBase64Dto, 'payload', 'Zm9v');
+        });
+
+        it('accepts base64 with single padding character', () => {
+            accepts(UnboundedBase64Dto, 'payload', 'YQ==');
+        });
+
+        it('accepts base64 with double padding', () => {
+            accepts(UnboundedBase64Dto, 'payload', 'YWI=');
+        });
+
+        it('accepts uppercase and lowercase alphabet characters', () => {
+            accepts(UnboundedBase64Dto, 'payload', toBase64('Ab+/Zz09'));
+        });
+
+        it('accepts plus and slash in the alphabet portion', () => {
+            const raw = Buffer.from([0xff, 0xfe, 0xfd, 0xfc, 0xfb, 0xfa]);
+
+            accepts(UnboundedBase64Dto, 'payload', raw.toString('base64'));
+        });
+
+        it('accepts large binary payloads', () => {
+            accepts(UnboundedBase64Dto, 'payload', toBase64(Buffer.alloc(4096, 0xab)));
+        });
+
+        it('accepts a one-byte decoded payload', () => {
+            accepts(UnboundedBase64Dto, 'payload', Buffer.from([0x00]).toString('base64'));
+        });
+
+        it.each([null, undefined, 0, 1, false, true, {}, [], Symbol('x')])(
+            'rejects non-string value %p',
+            value => {
+                rejects(UnboundedBase64Dto, 'payload', value);
+            }
+        );
+
+        it('rejects empty string', () => {
+            rejects(UnboundedBase64Dto, 'payload', '');
+        });
+
+        it.each(['@', '#', '$', '%', '-', '_', '.', ',', '!', '?', '\\', '"', "'"])(
+            'rejects disallowed character %s',
+            char => {
+                rejects(UnboundedBase64Dto, 'payload', `YQ==${char}`);
+            }
+        );
+
+        it('rejects whitespace before valid base64', () => {
+            rejects(UnboundedBase64Dto, 'payload', ` ${toBase64('a')}`);
+        });
+
+        it('rejects whitespace after valid base64', () => {
+            rejects(UnboundedBase64Dto, 'payload', `${toBase64('a')} `);
+        });
+
+        it('rejects embedded newline', () => {
+            rejects(UnboundedBase64Dto, 'payload', 'YQ==\n');
+        });
+
+        it('rejects embedded tab', () => {
+            rejects(UnboundedBase64Dto, 'payload', 'YQ\t==');
+        });
+
+        it('rejects unicode outside base64 alphabet', () => {
+            rejects(UnboundedBase64Dto, 'payload', 'YQé==');
+        });
+
+        it('rejects more than two padding characters', () => {
+            rejects(UnboundedBase64Dto, 'payload', 'YQ===');
+        });
+
+        it('rejects padding characters in the middle of the string', () => {
+            rejects(UnboundedBase64Dto, 'payload', 'Y=Q=');
+        });
+
+        it('rejects strings that are only padding', () => {
+            rejects(UnboundedBase64Dto, 'payload', '==');
+        });
+
+        it('returns the generic error message', () => {
+            const errors = validateField(UnboundedBase64Dto, 'payload', 'not!!!valid');
+
+            expect(constraintMessage(errors, 'payload')).toBe('payload must be a valid base64 string');
+        });
+    });
+
+    describe('with byteLength', () => {
+        const twelveBytes = Buffer.alloc(12, 0x11);
+        const twelveBytesB64 = twelveBytes.toString('base64');
+
+        it('accepts base64 that decodes to exactly 12 bytes', () => {
+            accepts(TwelveByteBase64Dto, 'iv', twelveBytesB64);
+        });
+
+        it('accepts 12 random bytes', () => {
+            const iv = Buffer.from([
+                0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c
+            ]);
+
+            accepts(TwelveByteBase64Dto, 'iv', iv.toString('base64'));
+        });
+
+        it('accepts 32-byte secrets when byteLength is 32', () => {
+            accepts(ThirtyTwoByteBase64Dto, 'secretKey', Buffer.alloc(32, 0x42).toString('base64'));
+        });
+
+        it('rejects valid base64 that decodes to 11 bytes when 12 are required', () => {
+            rejects(TwelveByteBase64Dto, 'iv', Buffer.alloc(11, 0x01).toString('base64'));
+        });
+
+        it('rejects valid base64 that decodes to 13 bytes when 12 are required', () => {
+            rejects(TwelveByteBase64Dto, 'iv', Buffer.alloc(13, 0x01).toString('base64'));
+        });
+
+        it('rejects valid base64 that decodes to 1 byte when 32 are required', () => {
+            rejects(ThirtyTwoByteBase64Dto, 'secretKey', toBase64('a'));
+        });
+
+        it('rejects valid base64 that decodes to 31 bytes when 32 are required', () => {
+            rejects(ThirtyTwoByteBase64Dto, 'secretKey', Buffer.alloc(31, 0x01).toString('base64'));
+        });
+
+        it('rejects invalid base64 even when length would have matched', () => {
+            rejects(TwelveByteBase64Dto, 'iv', `${twelveBytesB64}!`);
+        });
+
+        it('rejects empty string when byteLength is set', () => {
+            rejects(TwelveByteBase64Dto, 'iv', '');
+        });
+
+        it('returns byteLength-specific error message', () => {
+            const errors = validateField(TwelveByteBase64Dto, 'iv', toBase64('short'));
+
+            expect(constraintMessage(errors, 'iv')).toBe('iv must be a base64-encoded 12-byte value');
+        });
+
+        it('returns byteLength-specific error message for 32-byte constraint', () => {
+            const errors = validateField(ThirtyTwoByteBase64Dto, 'secretKey', toBase64('x'));
+
+            expect(constraintMessage(errors, 'secretKey')).toBe(
+                'secretKey must be a base64-encoded 32-byte value'
+            );
+        });
+    });
+
+    describe('multiple decorated properties on one DTO', () => {
+        const iv = Buffer.alloc(12, 0x99).toString('base64');
+        const ciphertext = toBase64(Buffer.alloc(64, 0xcd));
+
+        it('accepts when both iv and ciphertext are valid', () => {
+            const dto = Object.assign(new DualFieldDto(), { iv, ciphertext });
+
+            expect(validateSync(dto)).toHaveLength(0);
+        });
+
+        it('rejects when only iv is invalid', () => {
+            const dto = Object.assign(new DualFieldDto(), { iv: 'bad', ciphertext });
+
+            const errors = validateSync(dto);
+
+            expect(errors).toHaveLength(1);
+            expect(errors[0].property).toBe('iv');
+        });
+
+        it('rejects when only ciphertext is invalid', () => {
+            const dto = Object.assign(new DualFieldDto(), { iv, ciphertext: '###' });
+
+            const errors = validateSync(dto);
+
+            expect(errors).toHaveLength(1);
+            expect(errors[0].property).toBe('ciphertext');
+        });
+
+        it('reports both properties when both are invalid', () => {
+            const dto = Object.assign(new DualFieldDto(), { iv: '', ciphertext: '' });
+
+            const errors = validateSync(dto);
+
+            expect(errors.map(e => e.property).sort()).toEqual(['ciphertext', 'iv']);
+        });
+    });
+
+    describe('default options object', () => {
+        class DefaultOptionsDto {
+            @IsBase64({})
+            payload: string;
+        }
+
+        it('behaves like unbounded base64 when options is an empty object', () => {
+            accepts(DefaultOptionsDto, 'payload', toBase64('defaults'));
+        });
+
+        it('rejects wrong byte length only when byteLength is explicitly set', () => {
+            rejects(TwelveByteBase64Dto, 'iv', toBase64('x'));
+            accepts(UnboundedBase64Dto, 'payload', toBase64('x'));
+        });
+    });
+});
diff --git a/backend/src/validation/decorators/isBase64.ts b/backend/src/validation/decorators/isBase64.ts
new file mode 100644
index 0000000..b2cdecf
--- /dev/null
+++ b/backend/src/validation/decorators/isBase64.ts
@@ -0,0 +1,48 @@
+import {
+    Validate,
+    ValidatorConstraint,
+    type ValidationArguments,
+    type ValidatorConstraintInterface
+} from 'class-validator';
+import type { IsBase64Options } from '../../types/validation/IsBase64Options';
+
+const BASE64_PATTERN = /^[A-Za-z0-9+/]+={0,2}$/;
+
+@ValidatorConstraint({ name: 'isBase64' })
+class IsBase64Constraint implements ValidatorConstraintInterface {
+    validate(value: unknown, args: ValidationArguments): boolean {
+        if (typeof value !== 'string' || !value) {
+            return false;
+        }
+
+        if (!BASE64_PATTERN.test(value)) {
+            return false;
+        }
+
+        const decoded = Buffer.from(value, 'base64');
+
+        if (decoded.length === 0) {
+            return false;
+        }
+
+        const [{ byteLength }] = args.constraints as [IsBase64Options];
+
+        if (byteLength !== undefined) {
+            return decoded.length === byteLength;
+        }
+
+        return true;
+    }
+
+    defaultMessage(args: ValidationArguments): string {
+        const [{ byteLength }] = args.constraints as [IsBase64Options];
+
+        if (byteLength !== undefined) {
+            return `$property must be a base64-encoded ${byteLength}-byte value`;
+        }
+
+        return '$property must be a valid base64 string';
+    }
+}
+
+export const IsBase64 = (options: IsBase64Options = {}) => Validate(IsBase64Constraint, [options]);
diff --git a/backend/src/validation/decorators/nullOr.spec.ts b/backend/src/validation/decorators/nullOr.spec.ts
new file mode 100644
index 0000000..31037df
--- /dev/null
+++ b/backend/src/validation/decorators/nullOr.spec.ts
@@ -0,0 +1,57 @@
+import 'reflect-metadata';
+import { plainToInstance } from 'class-transformer';
+import { validateSync } from 'class-validator';
+import { NullOrDate, NullOrInt } from './nullOr';
+
+class DateFieldDto {
+    @NullOrDate()
+    expiresAt: Date | null;
+}
+
+class IntFieldDto {
+    @NullOrInt({ min: 1, max: 100 })
+    maxViews: number | null;
+}
+
+describe('NullOrDate', () => {
+    it('accepts null', () => {
+        const dto = plainToInstance(DateFieldDto, { expiresAt: null });
+
+        expect(validateSync(dto)).toHaveLength(0);
+    });
+
+    it('accepts an ISO string transformed to Date', () => {
+        const dto = plainToInstance(DateFieldDto, { expiresAt: '2026-06-01T00:00:00.000Z' });
+
+        expect(validateSync(dto)).toHaveLength(0);
+        expect(dto.expiresAt).toBeInstanceOf(Date);
+    });
+
+    it('rejects missing property', () => {
+        expect(validateSync(new DateFieldDto()).length).toBeGreaterThan(0);
+    });
+});
+
+describe('NullOrInt', () => {
+    it('accepts null', () => {
+        const dto = Object.assign(new IntFieldDto(), { maxViews: null });
+
+        expect(validateSync(dto)).toHaveLength(0);
+    });
+
+    it('accepts an integer within bounds', () => {
+        const dto = Object.assign(new IntFieldDto(), { maxViews: 50 });
+
+        expect(validateSync(dto)).toHaveLength(0);
+    });
+
+    it('rejects out-of-range integers', () => {
+        const dto = Object.assign(new IntFieldDto(), { maxViews: 0 });
+
+        expect(validateSync(dto).length).toBeGreaterThan(0);
+    });
+
+    it('rejects missing property', () => {
+        expect(validateSync(new IntFieldDto()).length).toBeGreaterThan(0);
+    });
+});
diff --git a/backend/src/validation/decorators/nullOr.ts b/backend/src/validation/decorators/nullOr.ts
new file mode 100644
index 0000000..d544290
--- /dev/null
+++ b/backend/src/validation/decorators/nullOr.ts
@@ -0,0 +1,77 @@
+import { applyDecorators } from '@nestjs/common';
+import { Type } from 'class-transformer';
+import {
+    Validate,
+    ValidatorConstraint,
+    type ValidationArguments,
+    type ValidationOptions,
+    type ValidatorConstraintInterface
+} from 'class-validator';
+import type { NullOrIntOptions } from '../../types/validation/NullOrIntOptions';
+
+@ValidatorConstraint({ name: 'nullOrDate' })
+class NullOrDateConstraint implements ValidatorConstraintInterface {
+    validate(value: unknown): boolean {
+        if (value === null) {
+            return true;
+        }
+
+        return value instanceof Date && !Number.isNaN(value.getTime());
+    }
+
+    defaultMessage(): string {
+        return '$property must be null or a valid date';
+    }
+}
+
+@ValidatorConstraint({ name: 'nullOrInt' })
+class NullOrIntConstraint implements ValidatorConstraintInterface {
+    validate(value: unknown, args: ValidationArguments): boolean {
+        if (value === null) {
+            return true;
+        }
+
+        const [{ min, max }] = args.constraints as [NullOrIntOptions];
+
+        if (typeof value !== 'number' || !Number.isInteger(value)) {
+            return false;
+        }
+
+        if (min !== undefined && value < min) {
+            return false;
+        }
+
+        if (max !== undefined && value > max) {
+            return false;
+        }
+
+        return true;
+    }
+
+    defaultMessage(args: ValidationArguments): string {
+        const [{ min, max }] = args.constraints as [NullOrIntOptions];
+
+        if (min !== undefined && max !== undefined) {
+            return `$property must be null or an integer between ${min} and ${max}`;
+        }
+
+        if (min !== undefined) {
+            return `$property must be null or an integer >= ${min}`;
+        }
+
+        if (max !== undefined) {
+            return `$property must be null or an integer <= ${max}`;
+        }
+
+        return '$property must be null or an integer';
+    }
+}
+
+export const NullOrDate = (validationOptions?: ValidationOptions) =>
+    applyDecorators(
+        Type(() => Date),
+        Validate(NullOrDateConstraint, validationOptions)
+    );
+
+export const NullOrInt = (options?: NullOrIntOptions, validationOptions?: ValidationOptions) =>
+    applyDecorators(Validate(NullOrIntConstraint, [options ?? {}], validationOptions));